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.

tests/pull-test.sh

@@ -55,10 +55,10 @@ function verify_initial_contents() {
 }
 
 if has_gpgme; then
-    echo "1..36"
+    echo "1..37"
 else
     # 3 tests needs GPG support
-    echo "1..33"
+    echo "1..34"
 fi
 
 # Try both syntaxes
@@ -142,6 +142,14 @@ ${CMD_PREFIX} ostree --repo=mirrorrepo pull origin main
 ${CMD_PREFIX} ostree --repo=mirrorrepo fsck
 echo "ok pull (refuses deltas)"
 
+${CMD_PREFIX} ostree --repo=mirrorrepo remote add broken badscheme://something
+if ${CMD_PREFIX} ostree --repo=mirrorrepo pull broken main 2>err.txt; then
+  assert_not_reached "pulled from invalid"
+fi
+assert_file_has_content_literal err.txt "Invalid URI scheme in badscheme://something"
+${CMD_PREFIX} ostree --repo=mirrorrepo remote delete broken
+echo "ok clean error on invalid scheme"
+
 cd ${test_tmpdir}
 rm mirrorrepo/refs/remotes/* -rf
 ${CMD_PREFIX} ostree --repo=mirrorrepo prune --refs-only