CatalogStackBase._validate_catalog_url read parsed.hostname, which raises
ValueError on a malformed authority (e.g. an unclosed ipv6 bracket
"https://[::1"). every other reject path raises the class catalog error, so
the raw ValueError leaked to the caller. wrap the parse + hostname access and
convert ValueError to the normal error via cls._error.
twin of the bundler fix in adapters._validate_remote_url.
