Both lcow_parser.go and linux_parser.go are duplicating the error:
"invalid specification: destination can't be '/'"
This commit creates a new error called "ErrVolumeTargetIsRoot"
that is used by both linux_parser and lcow_parser and remove
the duplication in the code.
Signed-off-by: Boaz Shuster <ripcurld.github@gmail.com>