Before this change the Errors type in the union backend produced
errors which could not be Unwrapped to test their type.
This adds the (go1.20) Unwrap method to the Errors type which allows
errors.Is to work on these errors.
It also adds unit tests for the Errors type and fixes a couple of
minor bugs thrown up in the process.
See: https://forum.rclone.org/t/failed-to-set-modification-time-1-error-pcloud-cant-set-modified-time/38596
