LWC compiler should warn/error when handler is missing - or better runtime error msg

[midzelis]

Description

When handler doesn't exist, LWC should produce warning or error. Happens often if you refactor or because of typos.

Steps to Reproduce

https://playground.sfdc.es/projects/ByYXVjbk7/2/edit

<template>
    <input onchange={handler}></input>
</template>

Expected Results

Compiler error message, or better runtime error.

Actual Results

Poor runtime error only.

Browsers Affected

Version

• LWC: x.x.x

Possible Solution

Additional context/Screenshots
Add any other context about the problem here. If applicable, add screenshots to help explain.

[caridy]

We now have a better error, but can't be a compile error due to inheritance, so, it is a runtime error.