Before this change, all types of checkers showed "checking" after the
file name despite the fact that not all of them were checking.
After this change, they can show
- checking
- deleting
- hashing
- importing
- listing
- merging
- moving
- renaming
See: https://forum.rclone.org/t/what-is-rclone-checking-during-a-purge/35931/
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.