convert RequestException into a more standard NonfatalArtifactCacheError
convert RequestException into a more standard NonfatalArtifactCacheError

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/125596147

Bugs closed: 3266

Reviewed at https://rbcommons.com/s/twitter/r/3754/

landism/pants,lahosken/pants,baroquebobcat/pants,wisechengyi/pants,mateor/pants,peiyuwang/pants,benjyw/pants,fkorotkov/pants,ity/pants,wisechengyi/pants,twitter/pants,landism/pants,pantsbuild/pants,cevaris/pants,baroquebobcat/pants,15Dkatz/pants,jsirois/pants,ericzundel/pants,cevaris/pants,manasapte/pants,mateor/pants,tdyas/pants,wisechengyi/pants,fkorotkov/pants,pantsbuild/pants,gmalmquist/pants,baroquebobcat/pants,kwlzn/pants,pantsbuild/pants,manasapte/pants,fkorotkov/pants,gmalmquist/pants,jsirois/pants,fkorotkov/pants,mateor/pants,ity/pants,pantsbuild/pants,ericzundel/pants,pantsbuild/pants,manasapte/pants,lahosken/pants,benjyw/pants,mateor/pants,cevaris/pants,twitter/pants,UnrememberMe/pants,UnrememberMe/pants,tdyas/pants,ericzundel/pants,kwlzn/pants,manasapte/pants,mateor/pants,pombredanne/pants,benjyw/pants,baroquebobcat/pants,pombredanne/pants,tdyas/pants,foursquare/pants,cevaris/pants,15Dkatz/pants,manasapte/pants,wisechengyi/pants,lahosken/pants,tdyas/pants,ericzundel/pants,ity/pants,benjyw/pants,benjyw/pants,twitter/pants,15Dkatz/pants,15Dkatz/pants,fkorotkov/pants,cevaris/pants,foursquare/pants,kwlzn/pants,tdyas/pants,lahosken/pants,ericzundel/pants,benjyw/pants,UnrememberMe/pants,ericzundel/pants,peiyuwang/pants,baroquebobcat/pants,ity/pants,UnrememberMe/pants,UnrememberMe/pants,lahosken/pants,UnrememberMe/pants,gmalmquist/pants,twitter/pants,ericzundel/pants,landism/pants,landism/pants,twitter/pants,gmalmquist/pants,peiyuwang/pants,tdyas/pants,kwlzn/pants,mateor/pants,cevaris/pants,peiyuwang/pants,baroquebobcat/pants,kwlzn/pants,foursquare/pants,jsirois/pants,fkorotkov/pants,pombredanne/pants,manasapte/pants,tdyas/pants,pantsbuild/pants,pombredanne/pants,lahosken/pants,mateor/pants,landism/pants,twitter/pants,foursquare/pants,kwlzn/pants,fkorotkov/pants,ericzundel/pants,gmalmquist/pants,twitter/pants,ity/pants,peiyuwang/pants,wisechengyi/pants,pombredanne/pants,foursquare/pants,twitter/pants,foursquare/pants,tdyas/pants,landism/pants,baroquebobcat/pants,gmalmquist/pants,ity/pants,15Dkatz/pants,pombredanne/pants,peiyuwang/pants,15Dkatz/pants,benjyw/pants,15Dkatz/pants,lahosken/pants,cevaris/pants,UnrememberMe/pants,kwlzn/pants,landism/pants,peiyuwang/pants,pantsbuild/pants,mateor/pants,foursquare/pants,tdyas/pants,15Dkatz/pants,wisechengyi/pants,foursquare/pants,lahosken/pants,UnrememberMe/pants,ity/pants,wisechengyi/pants,twitter/pants,wisechengyi/pants,baroquebobcat/pants,wisechengyi/pants,gmalmquist/pants,baroquebobcat/pants,pombredanne/pants,fkorotkov/pants,manasapte/pants,peiyuwang/pants,foursquare/pants,UnrememberMe/pants,landism/pants