Prior to this commit, if a user attempted to use the `vagrant snapshot
save` or `vagrant snapshot list` commands on a vm whose provider did not
support snapshots, it would simply print a warning. This commit changes
that behavior by instead raising an error.
Prior to this commit, the vagrant snapshot plugin would save snapshots
with existing names which lead to duplicate snapshot names being saved.
This commit fixes that by checking to see if the given snapshot name
already exists and if so, fails telling the user the given snapshot name
already exists. If a user passes a --force flag, vagrant will first
delete the existing snapshot, and take a new one with the given name.
Brian Cain