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/embedder_traits/resources.rs

@@ -61,6 +61,7 @@ pub enum Resource {
     RippyPNG,
     MediaControlsCSS,
     MediaControlsJS,
+    CrashHTML,
 }
 
 pub trait ResourceReaderMethods {
@@ -96,6 +97,7 @@ fn resources_for_tests() -> Box<dyn ResourceReaderMethods + Sync + Send> {
                 Resource::RippyPNG => "rippy.png",
                 Resource::MediaControlsCSS => "media-controls.css",
                 Resource::MediaControlsJS => "media-controls.js",
+                Resource::CrashHTML => "crash.html",
             };
             let mut path = env::current_exe().unwrap();
             path = path.canonicalize().unwrap();