fix: exit non-zero on Mastodon post failure

The script was swallowing exceptions and exiting 0, so systemd
considered the service successful and never triggered OnFailure
to notify via Gitea.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

get_saint_of_the_day.py

@@ -7,6 +7,7 @@ Optionally posts to Mastodon with the --post flag.
 import argparse
 import csv
 import os
+import sys
 from datetime import datetime
 from pathlib import Path
 import urllib.parse
@@ -195,6 +196,8 @@ def main():
         print("No saint's day today (or today is a holiday/feast day).")
         return
 
+    post_errors = []
+
     for saint in saints:
         print(f"Saint: {saint['name']}")
         print(f"French: {saint['french_name']}")
@@ -216,9 +219,14 @@ def main():
                     print(f"  Posted! URL: {status['url']}")
                 except Exception as e:
                     print(f"  Error posting: {e}")
+                    post_errors.append(str(e))
 
         print()
 
+    if post_errors:
+        print(f"{len(post_errors)} post(s) failed", file=sys.stderr)
+        sys.exit(1)
+
 
 if __name__ == "__main__":
     main()