Fail the step when a clone fails with no recourse

Fixes #2671.  There are a lot of issues with git's error handling..
Fix failures where 'git clean' fails but could be clobbered

Fixes #2677.
Merge djmitche/buildbot:8/git-clone-failure (PR #1057)
Merge djmitche/buildbot:8/git-clean-fails (PR #1058)