When f.opt.MaxAge == 0, f.db is never set, however several methods later assume
it is set and attempt to access it, causing an invalid memory address error.
This change fixes the issue in a few spots (there may still be others I haven't
yet encountered.)
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.
