prevent trying to get a commit that can't exist

[jakecoffman]

The change in #5937 caused Dependabot to begin raising an exception inside of the rescue for repositories that had a branch that didn't exist inside of their dependabot.yml.

What is happening is:

• The clone raises a HelperSubprocessFailed
• It gets turned into RepoNotFound
• The file_fetcher_job's rescue tries to call mark_job_as_processed
• But first it tries to get the base_commit_sha which... doesn't exist! So it also raises.

This fixes two issues:

• By setting @base_commit_sha to "unknown" when we know the clone failed, we can avoid the second raise.
• By checking the error message for "remote brach X not found" we can raise the correct error so the UI is more helpful

[Nishnha]

Looks correct to me

Definitely an improvement over the error cascades we had earlier - reminds me of throw null in Java