Increase error tolerance

[powerboat9]

These two commits should

1. Allow gerris to prepare a partial upstreaming, after hitting a commit it can't upstream
2. Prevent gerris from breaking on "gccrs: " commits it can't find analogues to in our repo

I wasn't able to test-run this, but it compiled on my end after running a cargo update

[powerboat9]

@CohenArthur thoughts?

[powerboat9]

Now depends on #26

[powerboat9]

This gets us the first 4 commits that haven't been upstreamed yet, although I'd be cautious about merging the fourth as-is