Before this change, `Gitlab::Git::Conflict::Resolver#conflicts`
is meant to rescue `GRPC::FailedPrecondition` exception and re-raise
`Gitlab::Git::Conflict::Resolver::ConflictSideMissing`.

That wasn't working though as `wrapped_gitaly_errors` will rescue
it as a `GRPC::BadStatus` exception because that's the parent of
`GRPC::FailedPrecondition`.

That results to MR head diff not showing up when there are
conflicts with missing side.

This fix moves the rescue inside the `wrapped_gitaly_errors`
block so we can rescue appropriately.

Changelog: fixed