Fix a TODO in _QueuedJob

Rather than raising Exception use GenericError and explain a bit better
what happened.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

lib/jqueue.py

@@ -177,8 +177,7 @@ class _QueuedJob(object):
 
     """
     if not ops:
-      # TODO: use a better exception
-      raise Exception("No opcodes")
+      raise errors.GenericError("A job needs at least one opcode")
 
     self.queue = queue
     self.id = job_id