repo: Factor out the check of gpg result to a separate function

I plan to add a function for verifying any data which may return the
error about lack of trusted signatures, so let's avoid the redundancy
and put the check in the separate function.

Closes: #310
Approved by: cgwalters

apidoc/ostree-sections.txt

@@ -185,6 +185,7 @@ ostree_gpg_verify_result_get_all
 OstreeGpgSignatureFormatFlags
 ostree_gpg_verify_result_describe
 ostree_gpg_verify_result_describe_variant
+ostree_gpg_verify_result_require_valid_signature
 <SUBSECTION Standard>
 OSTREE_GPG_VERIFY_RESULT
 OSTREE_IS_GPG_VERIFY_RESULT