getdeps: improve debugging when failing to query tests from ctest
getdeps: improve debugging when failing to query tests from ctest

Summary:
We're seeing a failure on macOS in a CI environment but don't have much context:

```
Stderr:
+ GETDEPS=opensource/fbcode_builder/getdeps.py
+ opensource/fbcode_builder/getdeps.py test --facebook-internal watchman
Traceback (most recent call last):
  File "opensource/fbcode_builder/getdeps.py", line 436, in <module>
    sys.exit(main())
  File "opensource/fbcode_builder/getdeps.py", line 422, in main
    return args.func(args)
  File "opensource/fbcode_builder/getdeps.py", line 342, in run
    builder.run_tests(install_dirs, schedule_type=args.schedule_type)
  File "/data/sandcastle/boxes/trunk-hg-fbcode-fbsource/fbcode/opensource/fbcode_builder/getdeps/builder.py", line 372, in run_tests
    buck_test_info = list_tests()
  File "/data/sandcastle/boxes/trunk-hg-fbcode-fbsource/fbcode/opensource/fbcode_builder/getdeps/builder.py", line 352, in list_tests
    data = json.loads(output.decode("utf-8"))
  File "/opt/homebrew/Cellar/python27/2.7.10_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/opt/homebrew/Cellar/python27/2.7.10_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/opt/homebrew/Cellar/python27/2.7.10_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
```

Add some more context to that error message so we can learn more.

Reviewed By: pkaush

Differential Revision: D16061528

fbshipit-source-id: 23603a5d18651d20641ef1987b7094e73a9b1dbe

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro