Add response object in all MailChimpError exceptions

This allows any exception to be examined with more detail even when the response contains JSON data, by consistently including the response object

mailchimp3/mailchimpclient.py

@@ -23,6 +23,7 @@
 
 _logger = logging.getLogger('mailchimp3.client')
 
+
 def _enabled_or_noop(fn):
     @functools.wraps(fn)
     def wrapper(self, *args, **kwargs):
@@ -36,11 +37,19 @@ class MailChimpError(Exception):
 
 
 def _raise_response_error(r):
-    # in case of a 500 error, the response might not be a JSON
+    """
+    Return a MailChimpError to which we pass:
+        - the response object
+        - the response's JSON data if found.
+    """
+    error_data = {"response": r}
     try:
-        error_data = r.json()
+        json_data = r.json()
     except ValueError:
-        error_data = { "response": r }
+        # in case of a 500 error, the response might not be a JSON
+        pass
+    else:
+        error_data.update(json_data)
     raise MailChimpError(error_data)