Include result in exception

That gives slightly more information in case something goes wrong. However, it
currently only reports something like "errCode: 102", and we don't know what
that means.

This was motivated by issue #6.

growatt/__init__.py

@@ -144,5 +144,5 @@ def _back_success_response(self, response):
         data = response.json()
         result = data["back"]
         if not "success" in result or not result["success"]:
-            raise GrowattApiError()
+            raise GrowattApiError(result)
         return result