Errors thrown by storage drivers don't have the name of the driver, causing user
confusion about whether the error is coming from Docker or from a storage driver.
This change adds the storage driver name to each error message.
This required changing ErrUnsupportedDriver to a type, leading to code changes
whenever ErrUnsupportedDriver is used. The tests check whether the driver name
appears in the error message.
Signed-off-by: Amit Shukla <amit.shukla@docker.com>
9637cb40cd