Simplify error handling for UserCommitFiles RPC

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29598#note_192539828

• Extend the GRPC::InvalidArgument on the Gitaly side with a field to specify which argument is invalid
• Use that field to format a custom error message on the Rails side in Gitlab::Git::WrapsGitalyErrors, and if the field is not present return a generic message (e.g. Invalid argument, or Bad Request)
• Remove the commit existence validation added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29598