Send back UnsupportedEncoding exception as FailedPrecondition

https://sentry.gitlap.com/gitlab/gitlabcom-gitaly-ruby/issues/185219/

Gitlab::Git::Conflict::File::UnsupportedEncoding

This exception can be raised in ruby during ListConflictFiles. We should rescue this exception and translate it to grpc FailedPrecondition.

Also, in gitlab-ce, we should modify Gitlab::Git::Repository#can_be_merged? to rescue FailedPrecondition and return false.