Found by testing T7285: Okular: Improvement of error messages regarding signatures

To reproduce:

1. Kill gpg agent to make sure, there's no session open
2. Open and save a (smime or pgp) signed pdf file
3. Enter a wrong passphrase in pinentry 3 times ->
   • there's no error message
   • okular still shows "finish signing":
     
   • the file was written but has a broken signature:
     
     test_signed_11.pdf29 KBDownload