Before this change, Hasher did not check whether a "passed hash" (hashtype
natively supported by the wrapped backend) returned from a backend was blank,
and would sometimes return a blank hash to the caller even when a non-blank hash
was already stored in the db. This caused issues with, for example, Google
Drive, which has SHA1 / SHA256 hashes for some files but not others
(https://rclone.org/drive/#sha1-or-sha256-hashes-may-be-missing) and sometimes also
does not have hashes for very recently modified files.
After this change, Hasher will check if the received "passed hash" is
unexpectedly blank, and if so, it will continue to try other enabled methods,
such as retrieving a value from the database, or possibly regenerating it.
https://forum.rclone.org/t/hasher-with-gdrive-backend-does-not-return-sha1-sha256-for-old-files/44680/9?u=nielash
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.
