Before this change we tested special errors for straight equality.
This works for all normal backends, but the union backend may return
wrapped errors which contain the special error types.
In particular if a pcloud backend was part of a union when attempting
to set modification times the fs.ErrorCantSetModTime return wasn't
understood because it was wrapped in a union.Error.
This fixes the problem by using errors.Is instead in all the
comparisons in operations.
See: https://forum.rclone.org/t/failed-to-set-modification-time-1-error-pcloud-cant-set-modified-time/38596
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