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.
For some reason the API started returning some integers as strings in
JSON. This is probably OK in Javascript but it upsets Go.
This is easily fixed with the `json:"name,size"` struct tag.