Gracefully recover from mismatched SHA

Github webhook notifications sometimes get dropped, which can mean that homu is unaware of recent changes. We have [an assertion to guard against that](https://github.com/servo/homu/blob/d2b0d20a379cdf5f70b1cfd78f679c24b123cad4/homu/main.py#L1000), however that just causes silent failure. Instead, it should post a comment on the relevant PR asking for reapproval.