ExitError conditions now return a plain error with message like:
Error running git [config --system filter.lfs.clean git-lfs clean -- %f]: 'error: could not lock config file /etc/gitconfig: Permission denied' 'exit status 255'
Also:
* Made Set/Unset Config commands return errors when they occur.
* Made install attribute set fail properly with an error message e.g. if permission-denied. Previously this failed silently and reported success.
***NOTE*** this commit contains Go 1.6-specific APIs. The following commit will remove those, and that following commit can be reverted when Go >= 1.6 is required.
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