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`
+
 
 Older Releases
 --------------

docs/usage/sending_mail.rst

@@ -392,3 +392,15 @@ Exceptions
     `API error log <https://mandrillapp.com/settings/api>`_ to view the full API
     request and error response.)
 
+
+.. exception:: djrill.NotSerializableForMandrillError
+
+    The send call will raise a :exc:`~!djrill.NotSerializableForMandrillError` exception
+    if the message has attached data which cannot be serialized to JSON for the Mandrill API.
+
+    See :ref:`formatting-merge-data` for more information.
+
+    .. versionadded:: 2.0
+       Djrill 1.x raised a generic `TypeError` in this case.
+       :exc:`~!djrill.NotSerializableForMandrillError` is a subclass of `TypeError`
+       for compatibility with existing code.

docs/usage/templates.rst

@@ -75,6 +75,9 @@ which means advanced template users can include dicts and lists as merge vars
 (for templates designed to handle objects and arrays).
 See the Python :class:`json.JSONEncoder` docs for a list of allowable types.
 
+Djrill will raise :exc:`djrill.NotSerializableForMandrillError` if you attempt
+to send a message with non-json-serializable data.
+
 
 How To Use Default Mandrill Subject and From fields
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~