Need better error message when submodules are missing.

[dustymabe]

so just tried to use git-evtag to verify the latest tag from rpm-ostree and I'm seeing an issue:

$ rpm -q git-evtag
git-evtag-2016.1-1.fc22.x86_64
$ git checkout v2016.1
...
$ git-evtag verify v2016.1
gpg: Signature made Mon 28 Mar 2016 11:13:48 AM EDT using RSA key ID 21C13F0B
gpg: Good signature from "Colin Walters <walters@redhat.com>"
gpg:                 aka "Colin Walters <walters@verbum.org>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 1CEC 7A9D F7DA 85AB EF84  3DC0 A866 D7CC AE08 7291
     Subkey fingerprint: AB92 8A9C F8DD 0629 09C3  7BBD DC45 FD59 21C1 3F0B
error: Failed to resolve path '/home/dustymabe/Desktop/development/rpm-ostree/libglnx/.git': No such file or directory

Colin pointed out this is an issue because I haven't ran git submodule update --init. We need a better error message for this.