fix formatWebpackMessages to return all formatted messages instead of just one error.

Created by: onigoetz

Hi,

I propose this change to formatWebpackMessages that currently removes all errors but one to the places in which it is used.

I propose this change because it's really surprising that when you call a function that is supposed to format messages that actually formats and removes messages.

I moved the removal of messages where they're displayed. That makes it clearer why you see only one single error and not all the errors that occured.