drm/nouveau: improve reporting of fifo errors

Note: bar faults and semaphore errors were previously silently acked.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
drm/nouveau: report channel owner in ioctl error paths

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
drm/nouveau: report how much memory userspace tried to "validate"

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>