lxd-agent/exec: Improve support for detect fork exec errors
lxd-agent/exec: Improve support for detect fork exec errors

 - Detects missing parent directory and returns 127 exit status.
 - Detects permission denied and returns 126 exit status.

Also improves logging.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd