Include element tag name in callback error message

Show which element the {{on}} modifier is attached to, helping locate
the issue when modifiers are forwarded via ...attributes.

Author: johanrd

packages/@glimmer/runtime/lib/modifiers/on.ts

@@ -85,7 +85,7 @@ export class OnModifierState {
       throw new Error(
         `You must pass a function as the second argument to the \`on\` modifier; you passed ${
           userProvidedCallback === null ? 'null' : typeof userProvidedCallback
-        }. While rendering:\n\n${args.positional[1]?.debugLabel ?? `{unlabeled value}`}`
+        }. While rendering:\n\n${args.positional[1]?.debugLabel ?? '(unknown)'} on <${this.element.tagName.toLowerCase()}>`
       );
     }