Improve error handling loading custom hooks

Noticed this in kitchen.git, where my system Ruby was missing the
`berkshelf` gem, which is a LoadError rather than a NameError. Continue
on, but warn the user.

Change-Id: I0a0a21e27a1d6383857128cec418108fc40e3e7b
Reviewed-on: https://gerrit.causes.com/23021
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>