Fix error reporting when AuxErrorInfoPB is present without an error

As discussed in https://gerrit.cloudera.org/#/c/14824/24/be/src/runtime/query-driver.h@136, there is a bug in the error reporting path for query retries where it is possible for the error information set in TryQueryRetry is dropped. The impact is that the Status details (Status::AddDetail) set in TryQueryRetry won't get propagated to the client. This should only happen rarely, but it can only happen the ReportExecStatusRequestPB from a fragment contains a AuxErrorInfoPB entry, but no Status failure: https://gerrit.cloudera.org/#/c/14824/24/be/src/runtime/coordinator.cc@984