libostree: Actually trusted.gpg.d/*.gpg for GPG verification

The intent of this code I'm fairly certain was to use *.gpg from the
trusted.gpg.d, directory.  But right now, we're only using
"pubring.gpg" from that directory, which is odd.

Let's fix this to use all keys ending in .gpg, which will also
include pubring.gpg.
repo: Improve GPG error messages

The signing test is failing here on EL7 beta for me - it seems like
gnupg isn't honoring the homedir.