The error.html page now uses the resource_directory_url this
variable contains the relative path to /Base/res/ on the host
system as a file:// url. This is needed for future pages to load
resource files like icons. For the error.html page this was not
really needed because it lies over this own URL in FrameLoader.cpp.
This error page template is slightly hilarious and should probably
be replaced with AK::SourceGenerator or some such, but for now let's
just get rid of the call to String::format().
