Fix conflict resolver to handle FailedPrecondition error correctly

What does this MR do?

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.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Related to #281162 (closed)