Make rm error output more descriptive

[Drake-Eidukas]

While testing some of rm functionality, I came across this error:

drake@element:~/workspace/test_base$ cfg rm ~/workspace/test_base/testy.yaml 
error: 'git -C /home/drake/workspace/test_base "add changes"' failed with error:
exit status 1

It would be great to get more description on this error.

[Drake-Eidukas]

Meanwhile, cfg rm testy.yaml works without a hitch.