Treat LoadErrors for known gems as trivial

[denisdefreyne]

Fixes part of #1273.

• Known gems: e.g. require 'nokogiri' -> considered a trivial LoadError; Nanoc will tell you to add nokogiri to the Gemfile.

• Unknown gems: Usual stack trace.

[denisdefreyne]

The error message is still crappy:

% bundle exec nanoc
Loading site… Error: cannot load such file -- nokogiri
Make sure the gem is added to Gemfile and run `bundle install`.

Better:

% bundle exec nanoc
Loading site…

Error: cannot load such file -- nokogiri

1.  Add the `nokogiri` gem to your Gemfile:

        gem 'nokogiri'

2.  Run `bundle`

3.  Re-run `bundle exec nanoc`