First attempt for issue https://github.com/pantsbuild/pants/issues/326

End pants users arguably see no values from the stack trace (even if they occasionally do, they can use the reporting server log); but in PANTS_DEV mode, pants dev should see the stack. The change is to introduce a command line arg that pants bash script will pass to pants_exe if PANTS_DEV is set. And in pants_exe we setup an unhandled exception hook to print out the exception's stacktrace plus msg based on the cmdline arg's presence.

Testing Done:
17:04:04 05:01     [specs]
               Waiting for background workers to finish.
               SUCCESS
$ ./pants goal test tests::

Bugs closed: 326

Reviewed at https://rbcommons.com/s/twitter/r/655/