Make sure we've succesfully read()

If we've failed to register with the wproc manager, and also fail to
read the entire response, it makes more sense to output the value of
errno, since that might better indicate what's actually going on.

Besides, it's good practice to check your return values.

Signed-off-by: Anton Lofgren <alofgren@op5.com>
I was thinking more about using a log facility... But it seems to be in a core worker, so printf might be valid.