Print the traceback when there is a problem loading or calling a backend module
Print the traceback when there is a problem loading or calling a backend module

Testing Done:
I have a bug in my backend right now.  It was failing with:

```
devpants compile src/ --print-exception-stacktrace
*** Running pants in dev mode from /Users/zundel/Src/pants/src/python/pants/bin/pants_exe.py ***

Exception caught:
  File "/Users/zundel/Src/pants/src/python/pants/bin/pants_exe.py", line 71, in <module>
    main()
  File "/Users/zundel/Src/pants/src/python/pants/bin/pants_exe.py", line 66, in main
    _run()
  File "/Users/zundel/Src/pants/src/python/pants/bin/pants_exe.py", line 60, in _run
    goal_runner.setup()
  File "/Users/zundel/Src/pants/src/python/pants/bin/goal_runner.py", line 66, in setup
    build_configuration = load_plugins_and_backends(plugins, backend_packages)
  File "/Users/zundel/Src/pants/src/python/pants/base/extension_loader.py", line 30, in load_plugins_and_backends
    load_build_configuration_from_source(build_configuration, additional_backends=backends or [])
  File "/Users/zundel/Src/pants/src/python/pants/base/extension_loader.py", line 104, in load_build_configuration_from_source
    load_backend(build_configuration, backend_package)
  File "/Users/zundel/Src/pants/src/python/pants/base/extension_loader.py", line 126, in load_backend
    .format(backend=backend_module, error=e))

Exception message: Failed to load the pants.backend.core.register backend: cannot import name SourceSet
```

That's not much help. To debug it I had to comment out the exception handling in extension_loader.py.

After this change, it prints the (real) cause out  (circular import in my case)

```
Traceback (most recent call last):
  File "/Users/zundel/Src/pants/src/python/pants/base/extension_loader.py", line 122, in load_backend
    'register_goals'])
  File "/Users/zundel/Src/pants/src/python/pants/backend/core/register.py", line 12, in <module>
    from pants.backend.core.targets.doc import Page, Wiki, WikiArtifact
  File "/Users/zundel/Src/pants/src/python/pants/backend/core/targets/doc.py", line 11, in <module>
    from pants.backend.core.targets.source_set import SourceSet
  File "/Users/zundel/Src/pants/src/python/pants/backend/core/targets/source_set.py", line 14, in <module>
    from pants.backend.jvm.targets.jar_source_set import JarSourceSet
  File "/Users/zundel/Src/pants/src/python/pants/backend/jvm/targets/jar_source_set.py", line 11, in <module>
    from pants.backend.core.targets.source_set import SourceSet
ImportError: cannot import name SourceSet
```

Bugs closed: 921

Reviewed at https://rbcommons.com/s/twitter/r/1582/

jsirois/pants,kslundberg/pants,dgomez10/xanon,jessrosenfield/pants,dgomez10/xanon,slyphon/pants,benjyw/pants,TansyArron/pants,laurentgo/pants,qma/pants,peiyuwang/pants,manasapte/pants,Gabriel439/pants,tdyas/pants,pgroudas/pants,cevaris/pants,15Dkatz/pants,di0spyr0s/pants,di0spyr0s/pants,sid-kap/pants,mateor/pants,pantsbuild/pants,jtrobec/pants,manasapte/pants,UnrememberMe/pants,scode/pants,Gabriel439/pants,benjyw/pants,Gabriel439/pants,ity/pants,benjyw/pants,dturner-tw/pants,tejal29/pants,lahosken/pants,di0spyr0s/pants,laurentgo/pants,twitter/pants,sid-kap/pants,ericzundel/pants,kslundberg/pants,landism/pants,megaserg/pants,dbentley/pants,jessrosenfield/pants,jtrobec/pants,tdyas/pants,sameerparekh/pants,dgomez10/xanon,TansyArron/pants,digwanderlust/pants,qma/pants,mateor/pants,foursquare/pants,di0spyr0s/pants,peiyuwang/pants,ericzundel/pants,dgomez10/xanon,kslundberg/pants,peiyuwang/pants,lahosken/pants,baroquebobcat/pants,baroquebobcat/pants,scode/pants,dbentley/pants,jtrobec/pants,tdyas/pants,pombredanne/pants,fkorotkov/pants,Gabriel439/pants,tdyas/pants,fkorotkov/pants,pantsbuild/pants,dgomez10/xanon,jtrobec/pants,jessrosenfield/pants,laurentgo/pants,manasapte/pants,ity/pants,laurentgo/pants,15Dkatz/pants,areitz/pants,baroquebobcat/pants,ity/pants,15Dkatz/pants,dturner-tw/pants,jtrobec/pants,ericzundel/pants,kwlzn/pants,sameerparekh/pants,wisechengyi/pants,pantsbuild/pants,kslundberg/pants,tdyas/pants,slyphon/pants,lahosken/pants,dgomez10/xanon,baroquebobcat/pants,twitter/pants,scode/pants,mateor/pants,pantsbuild/pants,TansyArron/pants,scode/pants,digwanderlust/pants,manasapte/pants,peiyuwang/pants,ericzundel/pants,dgomez10/xanon,dbentley/pants,digwanderlust/pants,Gabriel439/pants,digwanderlust/pants,baroquebobcat/pants,sameerparekh/pants,dturner-tw/pants,qma/pants,TansyArron/pants,fkorotkov/pants,dgomez10/xanon,twitter/pants,landism/pants,kwlzn/pants,cevaris/pants,foursquare/pants,cevaris/pants,landism/pants,digwanderlust/pants,dbentley/pants,areitz/pants,foursquare/pants,peiyuwang/pants,twitter/pants,UnrememberMe/pants,laurentgo/pants,ity/pants,qma/pants,gmalmquist/pants,mateor/pants,slyphon/pants,dbentley/pants,gmalmquist/pants,areitz/pants,cevaris/pants,dturner-tw/pants,megaserg/pants,Gabriel439/pants,benjyw/pants,kslundberg/pants,UnrememberMe/pants,dturner-tw/pants,fkorotkov/pants,jessrosenfield/pants,megaserg/pants,scode/pants,UnrememberMe/pants,pgroudas/pants,UnrememberMe/pants,landism/pants,15Dkatz/pants,pombredanne/pants,foursquare/pants,UnrememberMe/pants,mateor/pants,pombredanne/pants,fkorotkov/pants,gmalmquist/pants,wisechengyi/pants,kwlzn/pants,mateor/pants,dbentley/pants,slyphon/pants,TansyArron/pants,areitz/pants,peiyuwang/pants,lahosken/pants,twitter/pants,15Dkatz/pants,mateor/pants,pantsbuild/pants,gmalmquist/pants,twitter/pants,baroquebobcat/pants,dgomez10/xanon,di0spyr0s/pants,di0spyr0s/pants,twitter/pants,qma/pants,sid-kap/pants,tejal29/pants,baroquebobcat/pants,lahosken/pants,ericzundel/pants,pombredanne/pants,gmalmquist/pants,cevaris/pants,15Dkatz/pants,tejal29/pants,15Dkatz/pants,foursquare/pants,cevaris/pants,sameerparekh/pants,kslundberg/pants,landism/pants,sid-kap/pants,sameerparekh/pants,manasapte/pants,kwlzn/pants,megaserg/pants,manasapte/pants,scode/pants,pgroudas/pants,ity/pants,benjyw/pants,wisechengyi/pants,pgroudas/pants,jtrobec/pants,lahosken/pants,dbentley/pants,Gabriel439/pants,landism/pants,scode/pants,baroquebobcat/pants,sid-kap/pants,baroquebobcat/pants,digwanderlust/pants,qma/pants,twitter/pants,wisechengyi/pants,pantsbuild/pants,TansyArron/pants,gmalmquist/pants,wisechengyi/pants,sameerparekh/pants,pombredanne/pants,mateor/pants,areitz/pants,kwlzn/pants,benjyw/pants,tdyas/pants,tdyas/pants,pombredanne/pants,dgomez10/xanon,jessrosenfield/pants,tdyas/pants,jtrobec/pants,megaserg/pants,wisechengyi/pants,pombredanne/pants,twitter/pants,kslundberg/pants,UnrememberMe/pants,foursquare/pants,foursquare/pants,foursquare/pants,megaserg/pants,jessrosenfield/pants,dturner-tw/pants,gmalmquist/pants,tejal29/pants,areitz/pants,jessrosenfield/pants,fkorotkov/pants,kwlzn/pants,jsirois/pants,ericzundel/pants,ity/pants,benjyw/pants,15Dkatz/pants,slyphon/pants,slyphon/pants,manasapte/pants,fkorotkov/pants,TansyArron/pants,landism/pants,ity/pants,tdyas/pants,foursquare/pants,ericzundel/pants,tejal29/pants,UnrememberMe/pants,qma/pants,dturner-tw/pants,sameerparekh/pants,peiyuwang/pants,sid-kap/pants,pgroudas/pants,di0spyr0s/pants,lahosken/pants,wisechengyi/pants,sid-kap/pants,laurentgo/pants,peiyuwang/pants,wisechengyi/pants,landism/pants,wisechengyi/pants,pantsbuild/pants,megaserg/pants,kwlzn/pants,cevaris/pants,UnrememberMe/pants,dgomez10/xanon,pgroudas/pants,fkorotkov/pants,slyphon/pants,areitz/pants,ericzundel/pants,jsirois/pants,lahosken/pants