[2.0.0-rc.1] Improve error message when function doesn't exist for v-on

[Async0x42]

For example: @chartclick="onChartClicked", if there is no: methods: { onChartClicked () { ...etc } }, then vuejs will report something like:

vue.common.js:1406 Uncaught TypeError: o.fn is not a function
(anonymous function) @ vue.common.js:1406
Vue.$emit @ vue.common.js:2142

I think there should be an error message as this isn't very clear that the method is missing.

[yyx990803]

There should be a "property does not exist" warning if you are using Chrome/FF/Edge. I've added a warning for with the event name too.

[Async0x42]

I'm using Chrome, but originally, I was only getting this exception being thrown with no warnings or messages.

If I recall correctly, I think I had seen a message about a property after trying various things, but normally I'd just get this exception thrown.

Thanks for the change!