Changed message to error_message, as that's what the API returns to m…

…e. Example JSON:

{"code": 400, "error_type": "OAuthException", "error_message": "Redirect URI doesn't match original redirect URI"}

instagram/oauth2.py

@@ -108,7 +108,7 @@ def exchange_for_access_token(self, code=None, username=None, password=None, sco
         response, content = http_object.request(url, method="POST", body=data)
         parsed_content = simplejson.loads(content)
         if int(response['status']) != 200:
-            raise OAuth2AuthExchangeError(parsed_content.get("message", ""))
+            raise OAuth2AuthExchangeError(parsed_content.get("error_message", ""))
         return parsed_content['access_token'], parsed_content['user']