linkcache: defer lookup exception failures if URL is mapped

It's possible to use rewriters to map input to an internally used
URL scheme. If that is the case, defer reporting an error
until a helper is called.

Signed-off-by: Jeff Mahoney <jeffm@jeffreymahoney.com>
wolframalphahelper: use URL mapping and handle results

The Wolfram Alpha helper was trying to catch an @calc directive that
would never be passed to it. Directives must be mapped via a rewriter
into a URL scheme. Since there's a direct translation to a URL, that's
easy enough to do.

Test cases are updated to match.

Signed-off-by: Jeff Mahoney <jeffm@jeffreymahoney.com>