Emit an early error message if tag or commit verification with git or
detached signature verification with gpg fails.
Make `verify_file_signature()` and `verify_git_signature()` return
non-zero in this case and set errors to `1`, so that later checks
in `check_pgpsigs()`, although still run, can not lead to a positive
result.
Signed-off-by: David Runge <dvzrv@archlinux.org>
3aa096a74f