Rollup plugin does not warn on parser warnings

[nolanlawson]

Create an LWC component like this:

<template>
  <slot></slot>
  <slot></slot>
</template>

This will create a warning due to a compiler warning for duplicate slots:

lwc/packages/@lwc/errors/src/compiler/error-info/template-transform.ts

Lines 516 to 521 in fc2fd37

	NO_DUPLICATE_SLOTS: {
	code: 1137,
	message: 'Invalid duplicate slot ({0}).',
	level: DiagnosticLevel.Warning,
	url: '',
	},

But as demonstrated in this minimal repro, it doesn't actually get logged anywhere on the command line when building with Rollup.

Adding a Rollup onwarn handler also doesn't log anything:

// rollup.config.js
export default {
  // ...
  onwarn(err) {
    console.error(err) // line never gets hit
  }
}

[uip-robot-zz]

This issue has been linked to a new work item: W-10930894