This adds the missing WSAECONNREFUSED error to the list of errors we
can retry under Windows.
> Connection refused. No connection could be made because the target
> computer actively refused it.
It also adds any relevant errors I could see in the error code list.
See: https://forum.rclone.org/t/failing-to-upload-large-file-to-b2/17085
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
