This commit adds some better handling around the snapshot restore and
delete commands for the virtualbox provider. If a user attempts to restore from
a vm that does not exist, instead of exiting 0 it will raise an
exception saying the virtual machine has not been created yet.
Addtionally, if a user attempts to restore from a snapshot id that does
not exist, instead of printing a complicated exception from the
virtualbox cli tool, it prints a more useful error message telling the
user that the snapshot does not exist.
Brian Cain