Try to be more helpful on container startup failure

This hides some of the confusing "command X failed to receive response"
why are usually caused by another more understandable error.

On failure to start() from lxc-start, a new error message is displayed,
suggesting the user sets logfile and loglevel and if using -d, restarts
the container in the foreground instead.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>