lib/gpg: Use nicer helper for gpg error messages

The vast majority of invocations of `ot_gpgme_error_to_gio_error()` were paired
with `g_prefix_error()`; let's combine them for the same reason we do
`glnx_throw_errno_prefix()`. For the few cases that don't we might as well add
some prefix.

I also changed it to `return FALSE` in prep for more style porting.

Closes: #1135
Approved by: jlebon

src/libostree/ostree-gpg-verify-result.c

@@ -133,8 +133,7 @@ ostree_gpg_verify_result_initable_init (GInitable     *initable,
   gpg_error = gpgme_new (&result->context);
   if (gpg_error != GPG_ERR_NO_ERROR)
     {
-      ot_gpgme_error_to_gio_error (gpg_error, error);
-      g_prefix_error (error, "Unable to create context: ");
+      ot_gpgme_throw (gpg_error, error, "Unable to create context");
       goto out;
     }