Prior to this commit, if a user specified that their `rsync__excludes`
option was an empty array, Vagrant would treat that as if it included
options inside the array rather than ignoring it. This commit fixes that
by only adding the excludes option when it exists and is not empty.
Rescue and re-wrap any errors encountered when running the post
rsync capability. Rescue this exception type and notify of error
when encountered by rsync auto. Include test coverage.
