(maint) Improve message during a load error in Rakefile

If the loading fails on "tasks/rails", it is generally because of a dependency
in one of the tasks. The current messaging won't give details of which require
failed in the task. This commit adds the message from the LoadError, which
gives details about which require failed.
Merge pull request #181 from haus/improve_rakefile_messaging

(maint) Improve message during a load error in Rakefile