cli: JS fixes
FileServer: enhance warning when no index.html file can be found
FileServer: respond with 'Not Found' instead of 'Internal Server Error'
FileServer: enhance returned error page for 404 response
FileServer: clarify how to use --fallback-page
amberc.js: update documentation
Merge pull request #733 from mkroehnert/cli

Amber CLI: JavaScript cleanup + enhanced error reporting for 'amber serve'