fix(json_in_md_parser): improve error messages for JSON parsing failures (#11948)

Signed-off-by: -LAN- <laipz8200@outlook.com>

api/libs/json_in_md_parser.py

@@ -27,7 +27,7 @@ def parse_json_markdown(json_string: str) -> dict:
         extracted_content = json_string[start_index:end_index].strip()
         parsed = json.loads(extracted_content)
     else:
-        raise Exception("Could not find JSON block in the output.")
+        raise ValueError("could not find json block in the output.")
 
     return parsed
 
@@ -36,10 +36,10 @@ def parse_and_check_json_markdown(text: str, expected_keys: list[str]) -> dict:
     try:
         json_obj = parse_json_markdown(text)
     except json.JSONDecodeError as e:
-        raise OutputParserError(f"Got invalid JSON object. Error: {e}")
+        raise OutputParserError(f"got invalid json object. error: {e}")
     for key in expected_keys:
         if key not in json_obj:
             raise OutputParserError(
-                f"Got invalid return object. Expected key `{key}` to be present, but got {json_obj}"
+                f"got invalid return object. expected key `{key}` to be present, but got {json_obj}"
             )
     return json_obj