Start factoring out common backend functionality

anymail/exceptions.py

@@ -1,3 +1,4 @@
+from django.core.exceptions import ImproperlyConfigured
 import json
 from requests import HTTPError
 
@@ -67,6 +68,10 @@ class AnymailError(Exception):
         return description
 
 
+class AnymailImproperlyConfigured(AnymailError, ImproperlyConfigured):
+    """Exception for configuration problems"""
+
+
 class AnymailAPIError(AnymailError):
     """Exception for unsuccessful response from ESP's API."""