Fail sync when encountering "N commits behind."

This is almost always something the user needs to address
before continuing work, so promoting it to a failure (rather
than simply an informational message) seems the right way to
go. As a side-effect, repo will now exit with a non-zero
status code in this situation, so pipelines of the form
`repo sync && make` will fail if there are branches that
are stalled due to uploaded but unmerged patches.

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