git command fails but build doesn't fail

[humitos]

Details

I noticed this while reporting #5032

• Read the Docs project URL: https://readthedocs.org/projects/test-builds/builds/
• Build URL (if applicable): https://readthedocs.org/projects/test-builds/builds/8319150/
• Read the Docs username (if applicable): humitos

Expected Result

Fail the build completely.

Actual Result

git command fails, but the build continues and passes. This means that the version built is invalid and we end up serving inconsistent docs.

[stsewd]

I saw this when svn was failing some weeks ago, we need to verify the exit code and raise the proper exception.