sm: Allow decryption even if expired other keys are configured.

* sm/gpgsm.c (main): Add special handling for bad keys in decrypt
mode.

The problem can easily be tested by adding --encrypt-to EXPIRED_KEY to
a decryption command. With that patch the errors are printed but
decryption continues and the process returns success unless other
errors occur.

• GnuPG-bug-id: T4431
• Signed-off-by: Werner Koch <wk@gnupg.org>