capability: on() without a function says what it got and what to pass

The message read "the handler of TurnOn is a function", which states the
opposite of what went wrong. It is now
  on("TurnOff") got undefined as the handler: pass a function that takes
  the context of the directive
under the endpoint and the interface, as every DeclarationError.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/dispatch/dispatch.test.js

@@ -139,6 +139,8 @@ test("on(): \"*\" and onDirective() take what has no handler of its own; a name
   assert.deepEqual(called, ["TurnOn TurnOn", "* TurnOff", "device SetBrightness"]);
   assert.throws(() => power.on("Toggle", by("Toggle")), (err) => err instanceof DeclarationError
     && err.message === "lamp-1: Alexa.PowerController: Toggle is not a directive of the interface, which has TurnOn, TurnOff");
+  assert.throws(() => power.on("TurnOff"), (err) => err instanceof DeclarationError
+    && err.message === "lamp-1: Alexa.PowerController: on(\"TurnOff\") got undefined as the handler: pass a function that takes the context of the directive");
 });
 
 test("a handler that throws or rejects answers INTERNAL_ERROR, the bridge reports what it threw, and nothing is left unhandled", async () => {