rpmSign() returned value 0 (success) in case of failure

- Fixes regression from commit c0aad81e9b17afcbea4008485d354673495eb148.
- That commit added line "res = replaceSignature(...)" that sets variable
  "res" to value 0. But error handling code following this line expects
  this variable set to -1. So in case of error value 0 (success) was
  returned but value -1 (failure) should have been returned.

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
rpmSign() always returned -1 if signatures was inserted into original rpm

- This was caused by commit 90833a57c523c1a5a074cae81c0c84fb3726fb6d but
  because of another problem fixed in previous commit it didn't exhibit.

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Error message with wrong filename was showed in case of error.

This was caused by commit 90833a57c523c1a5a074cae81c0c84fb3726fb6d.

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>