[IMP] component: improve error message when invalid handler

tests/components/__snapshots__/event_handling.test.ts.snap

@@ -1,5 +1,19 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`event handling Invalid handler throws an error 1`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component, comment } = bdom;
+  
+  let block1 = createBlock(\`<button block-handler-0=\\"click\\">click</button>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let hdlr1 = [ctx['dosomething'], ctx];
+    return block1([hdlr1]);
+  }
+}"
+`;
+
 exports[`event handling handler is not called if component is destroyed 1`] = `
 "function anonymous(bdom, helpers
 ) {

tests/components/event_handling.test.ts

@@ -33,6 +33,28 @@ describe("event handling", () => {
     expect(fixture.innerHTML).toBe("<span><div>simple vnode</div>2</span>");
   });
 
+  test("Invalid handler throws an error", async () => {
+    window.addEventListener(
+      "error",
+      (ev) => {
+        logStep(ev.error.message);
+        ev.preventDefault();
+      },
+      { once: true }
+    );
+
+    class Parent extends Component {
+      static template = xml`<button t-on-click="dosomething">click</button>`;
+
+      doSomething() {}
+    }
+
+    await mount(Parent, fixture);
+    expect([]).toBeLogged();
+    fixture.querySelector("button")!.click();
+    expect(["Invalid handler (expected a function, received: 'undefined')"]).toBeLogged();
+  });
+
   test("support for callable expression in event handler", async () => {
     class Counter extends Component {
       static template = xml`