make serverPrefetch() rejection trappable

[FranckFreiburger]

What problem does this feature solve?

Currently, rejections in serverPrefetch cannot be handled
eg.

serverPrefetch() {

   return Promise.reject('myError')
}

What does the proposed API look like?

Maybe send serverPrefetch rejections as renderStream error event ?

eg.

const renderStream = renderer.renderToStream(context)
renderStream.on('error', err => { ... })

[skyline0705]

Is there necessary to stop the renderer and throw an error for renderStream?
In Vue 2.6.x, I thinks this means "only this component has error, not the hold app",so it choose to stop render this component only, and the warning will be report when hydrating in browser.

[FranckFreiburger]

I agree that stopping the rendering is too severe for a rejection, but we need at least a mean to accurately detect that something is going wrong during a serverPrefetch.

AFAIU, serverPrefetch rejection arrives here (arguments[0] of end()):

vue/src/server/render-stream.js

Lines 44 to 49 in 6fe07eb

	this.end = () => {
	this.emit('beforeEnd')
	// the rendering is finished; we should push out the last of the buffer.
	this.done = true
	this.push(this.buffer)
	}

and could be simply pass as argument of this.emit('beforeEnd')

[CyberAP]

Just came across this. This issue actually prevents me from implementing renderToStream in my app at all because there's just no way of recovering from that state and for example refresh the page using renderToString. Really hope this could be fixed in Vue 2.7 and we'd get an error event if serverPrefetch fails.