* Improve tty error logging when buildkit vertex is unknown
Creates a generic "system" group in the tty output which captures
buildkit events that report a non-dagger vertex name. This happens
currently when using core.#Dockerfile actions since Dagger delegates the
LLB generation to buildkit through it's frontend and we don't get
meaningful events that we can correlate from Dagger's side
Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>
