auto retry on common errors

[Eh2406]

There are a lot of errors that are just going to lead to a reviewer giving a retry. It would be nice if the bot could do that automatically. For example rust-lang/rust#51855 (comment) where Travis failed to clone rust-lang/rust

We may be able to build this by #11 and having a custom message of

... Through arcane magic we have determined that this is spurious:
@ bors retry
...