* module/web/uri.scm (declare-default-port!): New function, declares a
default port for a scheme. Predeclare default ports for http and
https.
(unparse-uri): If the port is the default port for the given scheme,
don't serialize the port part of the URI.
* 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.