gpg: Make really sure that --verify-files always returns an error.

* g10/verify.c (verify_files): Track the first error code.

It seems to be possible to play tricks with packet structures so that
log_error is not used for a bad input data. By actually checking the
return code and let the main driver in gpg call log_error, we can fix
this case.

Note that using gpg --verify-files and relying solely on gpg's return
code is at best a questionable strategy. It is for example impossible
to tell which data has been signed.

• Signed-off-by: Werner Koch <wk@gnupg.org>