pull: Cleanly error out on unknown schemes

Previous to this we'd trip an assertion `abort()` deep in the curl code if e.g.
a user did `ostree remote add foo htttp://...` etc.

Motivated by considering supporting "external remotes" where code outside
ostree does a pull, but we want to reuse the signing verification infrastructure.

src/libostree/ostree-repo-pull.c

@@ -3446,6 +3446,9 @@ compute_effective_mirrorlist (OstreeRepo    *self,
       if (!baseuri)
         return FALSE;
 
+      if (!_ostree_fetcher_uri_validate (baseuri, error))
+        return FALSE;
+
       *out_mirrorlist =
         g_ptr_array_new_with_free_func ((GDestroyNotify) _ostree_fetcher_uri_free);
       g_ptr_array_add (*out_mirrorlist, g_steal_pointer (&baseuri));