tools: env: return error if ubi_update_start() fails

The UBI_IOCVOLUP ioctl can fail if exclusive access to the volume isn't
obtained. If this happens, the flush operation doesn't return error,
leaving the caller without knowledge of missing flush.

Fix this by forwarding the error (-1) from ubi_update_start().

Fixes: 34255b92 ("tools: env: Add support for direct read/write UBI volumes")
Signed-off-by: Martin Hundebøll <martin@geanix.com>