Grunt tasks: Better error report

[rxaviers]

Currently, when a grunt task fails (for instance, grunt deploy / grunt build-download), error output is so reduced (if not absent at all) which doesn't help to debug. For example: https://gist.github.com/scottgonzalez/25a2fdf8238d8c85d5fe

Can we make it more verbose?

[jzaefferer]

This is still not perfect (see #127), but overall much better.