add some TRACE/ERROR reporting

add some TRACE/ERROR reporting

The errors in execute_start are important because nothing actually prints
out what error if any there was in these cases, so you're left with an
empty log.

The TRACE logs are simply to tell you which version of start lxc chose to
invoke: exec or start.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>