error_num seems to be trying to remember the exit code of the init process,
except that nothing actually keeps track of it anywhere. So, let's add a
field to the handler, so that we can keep track of the process' exit
status, and the propagate it to error_num in struct lxc_container so that
people can use it.

Note that this is a slight behavior change, essentially instead of making
error_num always == the return code from start, now it contains slightly
more useful information (the actual exit status). But, there is only one
internal user of error_num which I'll fix in later in the series, so IMO
this is ok.
Signed-off-by: Tycho Andersen <tycho@tycho.ws>