google-calendar: Add error handling for missing client secret file.

Author: Niloth-p

zulip/integrations/google/get-google-credentials

@@ -1,5 +1,7 @@
 #!/usr/bin/env python3
+import logging
 import os
+import sys
 
 from google.auth.transport.requests import Request
 from google.oauth2.credentials import Credentials
@@ -40,9 +42,14 @@ def get_credentials() -> Credentials:
         if creds and creds.expired and creds.refresh_token:
             creds.refresh(Request())
         else:
-            flow = InstalledAppFlow.from_client_secrets_file(
-                os.path.join(HOME_DIR, CLIENT_SECRET_FILE), SCOPES
-            )
+            client_secret_path = os.path.join(HOME_DIR, CLIENT_SECRET_FILE)
+            if not os.path.exists(client_secret_path):
+                logging.error(
+                    "Unable to find the client secret file. Please ensure that you have downloaded the client secret file from Google, and placed it at %s.",
+                    client_secret_path,
+                )
+                sys.exit(1)
+            flow = InstalledAppFlow.from_client_secrets_file(client_secret_path, SCOPES)
             creds = flow.run_local_server(port=0)
         with open(tokens_path, "w") as token:
             token.write(creds.to_json())