Crash when accessing raw_path in check

[denisdefreyne]

(Summarise the bug in a single line.)

Steps to reproduce

Add this to the Checks file:

check :wat do
  @items.flat_map(&:reps).map(&:raw_path)
end

Run nanoc check wat.

Expected behavior

No crash.

Actual behavior

     FiberError:
       can't yield from root fiber

     # ./lib/nanoc/base/views/item_rep_view.rb:82:in `yield'
     # ./lib/nanoc/base/views/item_rep_view.rb:82:in `raw_path'
     # /private/var/folders/jf/v8gkrcc973l5l6f6561_zqp00000gn/T/nanoc-test20170320-41788-70bjge/Checks:2:in `map'