Before this change, bisync supported `--backup-dir` only when `Path1` and
`Path2` were different paths on the same remote. With this change, bisync
introduces new `--backup-dir1` and `--backup-dir2` flags to support separate
backup-dirs for `Path1` and `Path2`.
`--backup-dir1` and `--backup-dir2` can use different remotes from each other,
but `--backup-dir1` must use the same remote as `Path1`, and `--backup-dir2`
must use the same remote as `Path2`. Each backup directory must not overlap its
respective bisync Path without being excluded by a filter rule.
The standard `--backup-dir` will also work, if both paths use the same remote
(but note that deleted files from both paths would be mixed together in the
same dir). If either `--backup-dir1` and `--backup-dir2` are set, they will
override `--backup-dir`.
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.
