[625] Improve error reporting from generate

Right now the generate script exits if any of the routes throws an uncaught error. I noticed this when one of my routes bound to invalid html, like so:

<div v-html="invalidHTML"></div>

So the route crashes when attempting to minify the bad html. Rather than it exiting i would like generate to continue with the rest of my routes, then presenting me with an error report afterwards, like:

nuxt:generate Processed 318 routes. 314 HTML Files generated successfully in 74.2s +2ms

4 routes failed with errors:
/articles/author1.html
/articles/author45.html
/articles/author86.html
/articles/author89.html

I could implement this, but would like feedback about what to do about the stack traces. I can think of the following alternatives: 1. append to the above error report. that would make it much harder too read. 2. like npm, write an error log to a file at some clever location.