Allow custom handlers for HTTP errors (especially 404 and 500)

[agronholm]

If this can be done with current WebCore, I don't know how. Implement and document this.

[amcgregor]

This is actually demonstrated in the Paste Deploy Example, but needs to be explicitly documented.

[amcgregor]

See also #19 which has a simplified demonstration of how to do this.

[agronholm]

Using Paste's errorDocument for this, I noticed two problems:

1. I no longer get emails notifications for exceptions
2. web.url() produces incorrect application-relative URLs

[amcgregor]

Now easily possible by registering a view handler for the appropriate HTTPException subclass, amongst other methods (such as front-end web server configuration).