Cleanup Djrill exceptions

* Add common base DjrillException

* Simplify backend by moving logic
  to describe errors into base DjrillException

* Add NotSerializableForMandrillError
  for JSON serialization errors

docs/history.rst

@@ -88,6 +88,8 @@ Other Djrill 2.0 Changes
   (You can also directly manage your own long-lived Djrill connection across multiple sends,
   by calling open and close on :ref:`Django's email backend <django:topic-email-backends>`.)
 
+* Add :exc:`djrill.NotSerializableForMandrillError`
+
 
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