Check if job is not already done or cancelled when calling cancel method.

ccserver/jobs.py

@@ -151,6 +151,10 @@ class BaseJob(dict, Thread, object):
             the job.
         '''
 
+        if self['done']:
+            raise JobError('Job is done')
+        if self['cancelled']:
+            raise JobError('Job is already cancelled')
         self['cancelled'] = True
         self.report('cancelling')