operations: Fix error paths in CherryPick on custom hook error

While checking whether we can remove the PreReceiveError field of the UserCherryPick response I stumbled upon one branch where we seemingly still setting that field. As it turns out though we don't as the error condition is accidentally handled twice, and as a result we would always return the structured error as expected.

Fix the confused code by removing the redundant-and-wrong error handling.

Part of gitlab#375572 (closed).