constellation: crash to a new “sad tab” error page (#30290)

* constellation: crash to a new “sad tab” page

* check in resources/crash.html

* use a separate enum variant instead of keying on reason

* fmt + tidy

* rename Resource::Crash to Resource::CrashHTML

* clean up crash page and add details (reason + backtrace)

* avoid repeating crash errors in script::script_thread warn log

* make new LoadData init more idiomatic

* clarify comments and new fields

* fix doc comment style

components/net_traits/lib.rs

@@ -758,8 +758,10 @@ pub enum NetworkError {
     /// Could be any of the internal errors, like unsupported scheme, connection errors, etc.
     Internal(String),
     LoadCancelled,
-    /// SSL validation error that has to be handled in the HTML parser
+    /// SSL validation error, to be converted to Resource::BadCertHTML in the HTML parser.
     SslValidation(String, Vec<u8>),
+    /// Crash error, to be converted to Resource::Crash in the HTML parser.
+    Crash(String),
 }
 
 impl NetworkError {