Check reporting of Git errors

[martinmcclure]

Esteban Maringolo reports via the Pharo Users list that he had trouble using SETT because of Git errors. These particular errors were due to user.name and user.email global settings not being set up on the newly-installed Linux machine.

We should check to make sure that errors from Git commands being executed by SETT are being reported in a way that users can easily diagnose the problem.