MOB-8620: Map USB_CANCELED_ERROR to ERROR_CANCELLED

This commit updates Transfer::TranslateError() to report ERROR_CANCELLED,
rather than ERROR_INTERNAL_ERROR, for transfers that complete with
USB_CANCELED_ERROR. Cancelled transfers may still have successfully sent
or received some data over the USB, and clients should always check the
number of bytes transferred. Client documentation has been updated to make
this clear.