[10.x] Dispatch failed notification event

[dam1r89]

The event NotificationFailed existed but was never used.

There is a small difference in how the code will behave with this error caching. For example, before if the first channel failed, the rest of them would fail too, now others will be processed.

[dam1r89]

Actually, this could be a problem, misconfigured mail would silently fail, so maybe rethrow the error after the event has been dispatched?

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!