bugfix: throw exception if structured output parser doesn't get what it wants (#3044)

allows the user to catch the issue and handle it rather than failing
hard.

This happens more than you'd expect when using output parsers with
chatgpt, especially if the temp is anything but 0. Sometimes it doesn't
want to listen and just does its own thing.

langchain/output_parsers/structured.py

@@ -38,6 +38,12 @@ class StructuredOutputParser(BaseOutputParser):
         return STRUCTURED_FORMAT_INSTRUCTIONS.format(format=schema_str)
 
     def parse(self, text: str) -> Any:
+        if "```json" not in text:
+            raise OutputParserException(
+                f"Got invalid return object. Expected markdown code snippet with JSON "
+                f"object, but got:\n{text}"
+            )
+
         json_string = text.split("```json")[1].strip().strip("```").strip()
         try:
             json_obj = json.loads(json_string)