The vdev-start-response message should cause the
completion to fire, even in the error case.  Otherwise,
the user still gets no useful information and everything
is blocked until the timeout period.

Add some warning text to print out the invalid status
code to aid debugging, and propagate failure code.
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>