LibWeb: Handle WebContent process crashes gracefully :^)

The OOPWV will now detect WebContent process crashes/disconnections and
simply create a new WebContent process in its place. We also generate a
little error page with a link to the crashing URL so you can reload and
try again.

This a huge step forward for OOPWV since it now has a feature that IPWV
can never replicate. :^)

Userland/Libraries/LibWeb/WebContentClient.cpp

@@ -37,6 +37,12 @@ WebContentClient::WebContentClient(OutOfProcessWebView& view)
     handshake();
 }
 
+void WebContentClient::die()
+{
+    ASSERT(on_web_content_process_crash);
+    on_web_content_process_crash();
+}
+
 void WebContentClient::handshake()
 {
     auto response = send_sync<Messages::WebContentServer::Greet>(getpid());