Handle GitHub API rate limit

GitHub API has serious rate limit for tokenless queries.

So instead of failing with traceback just print info that GitHub
API rate limit was exceeded.

did/plugins/github.py

@@ -78,6 +78,12 @@ def search(self, query):
             try:
                 data = json.loads(response.text)["items"]
                 result.extend(data)
+            except KeyError:
+                if json.loads(response.text)["message"].startswith(
+                        "API rate limit exceeded"):
+                    raise ReportError(
+                        "GitHub API rate limit exceeded. "
+                        "Consider creating an access token.")
             except requests.exceptions.JSONDecodeError as error:
                 log.debug(error)
                 raise ReportError(f"GitHub JSON failed: {response.text}.")