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/fetch/methods.rs

@@ -195,6 +195,13 @@ pub async fn main_fetch(
     // Step 1.
     let mut response = None;
 
+    // Servo internal: return a crash error when a crash error page is needed
+    if let Some(ref details) = request.crash {
+        response = Some(Response::network_error(NetworkError::Crash(
+            details.clone(),
+        )));
+    }
+
     // Step 2.
     if request.local_urls_only {
         if !matches!(