[Tech debt] Split success and error return values in uworker_output.ErrorType

The [ErrorType](https://github.com/google/clusterfuzz/blob/6c26da13a748e7ba7012eb1f24f904281ef1da4b/src/clusterfuzz/_internal/protos/uworker_msg.proto#L354) enum under uworker_output holds both error and success values, which lead to a cludge in #4516 so we can track error rate in uworker_main.

TODO: refactor the code that touches this Enum so errors and successful return values can be told apart. without having to list the success values outside of the proto.