Report git's output when git fails

[exarkun]

Fixes #124

[codecov-io]

Codecov Report

Merging #125 into master will decrease coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   93.43%   93.29%   -0.15%     
==========================================
  Files          15       15              
  Lines         640      626      -14     
  Branches       65       65              
==========================================
- Hits          598      584      -14     
  Misses         26       26              
  Partials       16       16

Impacted Files	Coverage Δ
src/towncrier/check.py	94% <100%> (+0.66%)	⬆️
src/towncrier/test/test_check.py	100% <100%> (ø)	⬆️

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[hawkowl]

@exarkun could you merge this forward? I'm assuming that the create_project change would work in the other tests?

[exarkun]

@hawkowl Done