Continue checking even if the target branch is broken about ofborg

[grahamc]

I started hacking on this, however the mass rebuild checker is a bit sensitive in this area:

1. We need to be careful either it is safe to call or we don't call find_after on the outpathdiffer if find_before failed
2. We need to make sure a sensible error message is propagated to the master check if find_before fails, which right now might be complicated.

It might make sense to have the out path checks be a completely separate eval check, that might clean this up. However, it is unique to most of the rest in that it operates on the branch before and after the PR is applied.

However, in the mean time, I have changed the before checking to upload a gist upon failure, so a teeny bit of improvement.