Added an ErrorHTML function that allows for the simple creation of an appealing error page from any handler that needs to produce and HTML-based error page.
Added an ErrorHTML function that allows for the simple creation of
an appealing error page from any handler that needs to produce
and HTML-based error page.

TODO: create ErrorJson and ErrorPlain to support other types of
parameterized error output.

carbocation/go.graf,carbocation/go.graf,carbocation/go.graf