Allow html from any EmailMultiAlternatives (not just DjrillMessage)

Includes type-checking on alternative message part, and switches
to ValueError (rather than ImproperlyConfigured) for unacceptable
alternatives.

[Also fixes bug where fail_silently=True wasn't respected.]

Cherry-picked from: medmunds/Djrill@faf53a1a0
(and parts of medmunds/Djrill@62d48c5f)

djrill/mail/backends/djrill.py

@@ -26,7 +26,7 @@ class DjrillBackend(BaseEmailBackend):
     Mandrill API Email Backend
     """
 
-    def __init__(self, fail_silently=False, **kwargs):
+    def __init__(self, **kwargs):
         """
         Set the API key, API url and set the action url.
         """
@@ -71,8 +71,13 @@ class DjrillBackend(BaseEmailBackend):
         if getattr(message, "alternative_subtype", None):
             if message.alternative_subtype == "mandrill":
                 self._build_advanced_message_dict(message)
-                if message.alternatives:
-                    self._add_alternatives(message)
+        try:
+            if getattr(message, 'alternatives', None):
+                self._add_alternatives(message)
+        except ValueError:
+            if not self.fail_silently:
+                raise
+            return False
 
         djrill_it = requests.post(self.api_action, data=json.dumps({
             "key": self.api_key,
@@ -121,23 +126,29 @@ class DjrillBackend(BaseEmailBackend):
             "from_name": message.from_name,
             "tags": message.tags,
             "track_opens": message.track_opens,
+            "track_clicks": message.track_clicks
         })
 
 
     def _add_alternatives(self, message):
         """
-        There can be only one! ... alternative attachment.
+        There can be only one! ... alternative attachment, and it must be text/html.
 
         Since mandrill does not accept image attachments or anything other
         than HTML, the assumption is the only thing you are attaching is
         the HTML output for your email.
         """
         if len(message.alternatives) > 1:
-            raise ImproperlyConfigured(
-                "Mandrill only accepts plain text and html emails. Please "
-                "check the alternatives you have attached to your message.")
+            raise ValueError(
+                "Too many alternatives attached to the message. "
+                "Mandrill only accepts plain text and html emails.")
+
+        (content, mimetype) = message.alternatives[0]
+        if mimetype != 'text/html':
+            raise ValueError("Invalid alternative mimetype '%s'. "
+                             "Mandrill only accepts plain text and html emails."
+                             % mimetype)
 
         self.msg_dict.update({
-            "html": message.alternatives[0][0],
-            "track_clicks": message.track_clicks
+            "html": message.alternatives[0][0]
         })