Search should return user message if GPT response is not valid JSON

Previously would throw if GPT response is not valid JSON. Better to
return original message to use for search instead

src/khoj/processor/conversation/gpt.py

@@ -165,7 +165,12 @@ Q: {text}
     )
 
     # Extract, Clean Message from GPT's Response
-    questions = json.loads(response["choices"][0]["text"].strip(empty_escape_sequences))
+    response_text = response["choices"][0]["text"]
+    try:
+        questions = json.loads(response_text.strip(empty_escape_sequences))
+    except json.decoder.JSONDecodeError:
+        logger.warn(f"GPT returned invalid JSON. Set question to empty list.\n{response_text}")
+        questions = [text]
     logger.debug(f"Extracted Questions by GPT: {questions}")
     return questions