Before this fix `NewObject` could return a wrapped `fs.Object(nil)`
which caused a crash. This was caused by `wrapObject` returning a
`nil` `*Object` which was cast into an `fs.Object`.
This changes the interface of `wrapObject` so it returns an
`fs.Object` instead of a `*Object` and an error which must be checked.
This forces the callers to return a `nil` object rather than an
`fs.Object(nil)`.
See: https://forum.rclone.org/t/panic-in-hasher-when-mounting-with-vfs-cache-and-not-synced-data-in-the-cache/29697/11
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.
