Fix IndexError in LLM Reranking when handling malformed LLM responses (…

…#17353)

* Fix bug #17352 by catching errors for malformed LLM responses

* Fix formatting

llama-index-core/llama_index/core/indices/utils.py

@@ -116,8 +116,20 @@ def default_parse_choice_select_answer_fn(
             continue
         answer_nums.append(answer_num)
         # extract just the first digits after the colon.
-        _answer_relevance = re.findall(r"\d+", line_tokens[1].split(":")[1].strip())[0]
-        answer_relevances.append(float(_answer_relevance))
+        try:
+            _answer_relevance = re.findall(
+                r"\d+", line_tokens[1].split(":")[1].strip()
+            )[0]
+            answer_relevances.append(float(_answer_relevance))
+        except (IndexError, ValueError) as e:
+            if not raise_error:
+                continue
+            else:
+                raise ValueError(
+                    f"Invalid answer line: {answer_line}. "
+                    "Answer line must be of the form: "
+                    "answer_num: <int>, answer_relevance: <float>"
+                )
     return answer_nums, answer_relevances