message: [tf.data service] Improve error message on job_name conflicts.
  This error comes up quite a bit and is usually confusing to users. This CL updates the error to be more actionable and understandable.
diff: @@ -361,15 +361,21 @@ Status DataServiceDispatcherImpl::ValidateMatchingJob(
  std::string requested = ProcessingModeToString(processing_mode);
  std::string actual = ProcessingModeToString(job->processing_mode);
  return errors::FailedPrecondition(
  - "Found a job with name ", job_name, ", but the processing mode <",
  - actual, "> doesn't match the requested processing mode <", requested,
  - ">.");
  + "Tried to create a job with name ", job_name, " and processing_mode <",
  + requested,
  + "> but there is already an existing job with that name using "
  + "processing mode <",
  + actual, ">");
  }
  if (job->dataset_id != dataset_id) {
  return errors::FailedPrecondition(
  - "Found a job with name ", job_name, ", but the dataset id <",
  - job->dataset_id, "> doesn't match the requested dataset id <",
  - dataset_id, ">.");
  + "Tried to create a job with name ", job_name, " for dataset ",
  + dataset_id,
  + ", but there is already an existing job with that name for dataset ",
  + job->dataset_id,
  + ". This either means that you are distributing two different datasets "
  + "under the same job_name, or that your dataset is being constructed "
  + "non-deterministically.");
  }
  return Status::OK();
  }
  