Custom error pages for 404, 500 status codes

[GoogleCodeExporter]

This can be implemented either through Mongoose web server error handling 
mechanism, or through the OnLoadError() callback in CEF.

For now I see only use for it for 404 pages. But there might be other cases, 
like directory listing denial (500?). PHP errors can be handled via 
set_error_handler and set_exception_handler functions. Though we might support 
all 5xx status codes here as well.

Original issue reported on code.google.com by czarek.t...@gmail.com on 8 May 2014 at 11:28