[ImageDecoder] Ensure global promises are marked as handled

There's no reason to spam the global error handler when these
promises are unhandled, they are for convienence only.

Fixes #934

Author: dalecurtis

index.src.html

@@ -5899,14 +5899,16 @@
     1. Assign `true` to {{ImageDecoder/[[closed]]}}.
     2. Clear {{ImageDecoder/[[codec implementation]]}} and release associated
         [=system resources=].
-    3. If {{ImageDecoder/[[ImageTrackList]]}} is empty, reject
-        {{ImageTrackList/[[ready promise]]}} with |exception|. Otherwise
+    3. If {{ImageDecoder/[[ImageTrackList]]}} is empty, [=reject=]
+        {{ImageTrackList/[[ready promise]]}} with |exception| and [=mark
+        as handled|mark it as handled=]. Otherwise
         perform these steps,
         1. Remove all entries from {{ImageDecoder/[[ImageTrackList]]}}.
         2. Assign `-1` to {{ImageDecoder/[[ImageTrackList]]}}'s
             {{ImageTrackList/[[selected index]]}}.
-    4. If {{ImageDecoder/[[complete]]}} is false resolve
-        {{ImageDecoder/[[completed promise]]}} with |exception|.
+    4. If {{ImageDecoder/[[complete]]}} is false, [=reject=]
+        {{ImageDecoder/[[completed promise]]}} with |exception| and [=mark
+        as handled|mark it as handled=].
 
 ImageDecoderInit Interface {#imagedecoderinit-interface}
 --------------------------------------------------------