Before this change, bisync ignored the dryRun parameter (only when specified
via the rc.)
This change fixes the issue, so that the dryRun rc parameter is equivalent to
the --dry-run flag.
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.