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.
- setup correct path encoding (fixes backend test FsEncoding)
- ignore range option if file is empty (fixes VFS test TestFileReadAtZeroLength)
- cleanup stray files left after failed upload (fixes test FsPutError)
- rebase code on master, adapt backend for rclone context passing
- translate Siad errors to rclone native FS errors in sia errorHandler
- TestSia: return proper backend options from the script
- TestSia: use uptodate AntFarm image, nebulouslabs/siaantfarm is stale
