🐛 publish: fix exit status integer overflow

publish.py currently uses the number of failed verifications as exit
status. `sys.exit(failed)` Whenever the number of failed verification
attempts is divisible by 256 the return status is 0. exit status 0
however conveys that there were no errors, so as long as an attacker can
controll the number of verification failures they can use this to turn
off verification alltogether.

fdroidserver/publish.py

@@ -471,7 +471,7 @@ def main():
     if failed:
         logging.error(_('%d APKs failed to be signed or verified!') % failed)
         if options.error_on_failed:
-            sys.exit(failed)
+            sys.exit(1)
 
 
 if __name__ == "__main__":