* module/web/uri.scm (parse-authority): Allow empty authorities, so that
we accept URIs of the form, file:///etc/hosts.
* test-suite/tests/web-uri.test ("string->uri"): Add tests.
* module/web/uri.scm (uri-error): New proc, throws to 'uri-error.
(validate-uri, uri-decode, uri-encode): Use uri-error.
* test-suite/tests/web-uri.test: Update for uri-error.