The exception message, when shade fails, will contain much more
specific information about the failure if the exception is treated
as a string. The 'message' attribute alone is usually not helpful.
Do not attempt to attach an already attached volume.
Likewise, do not attempt to detach a volume that is not
attached.
This version adds support for check mode.
James Cammarata
Toshio Kuratomi
David Shrewsbury
Greg DeKoenigsberg
Monty Taylor