show error log of KG extraction (#2045)

### What problem does this PR solve?

### Type of change


- [x] Performance Improvement

graphrag/graph_extractor.py

@@ -142,6 +142,7 @@ class GraphExtractor:
                 total_token_count += token_count
                 if callback: callback(msg=f"{doc_index+1}/{total}, elapsed: {timer() - st}s, used tokens: {total_token_count}")
             except Exception as e:
+                if callback: callback("Knowledge graph extraction error:{}".format(str(e)))
                 logging.exception("error extracting graph")
                 self._on_error(
                     e,
@@ -184,6 +185,7 @@ class GraphExtractor:
             text = perform_variable_replacements(CONTINUE_PROMPT, history=history, variables=variables)
             history.append({"role": "user", "content": text})
             response = self._llm.chat("", history, gen_conf)
+            if response.find("**ERROR**") >=0: raise Exception(response)
             results += response or ""
 
             # if this is the final glean, don't bother updating the continuation flag