Gracefully handle Git errors

[dylanpyle]

If shelling out to Git fails, we'd prefer not to end up in a half-baked situation where the file was updated but not committed. A couple of specific cases we might want to handle:

• Detect ahead of time that the run permission wasn't provided; maybe just treat this as a warning and continue?
  • Though maybe a --no-git flag or something would make the "I actually really don't want to use git" case more explicit instead of inferring it from the lack of permission.
• git CLI not being installed locally in the first place
• Working directory is not a git repo

[dylanpyle]

Additional cases I just ran into:

• No user.email / user.name set (I configure this per repo)
• Tag already exists

[justinmchase]

This appears to be partially fixed by #9