Improve tty error logging when buildkit vertex is unknown (#2188)

* 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>

util/progressui/printer.go

@@ -11,10 +11,12 @@ import (
 	"github.com/tonistiigi/units"
 )
 
-const antiFlicker = 5 * time.Second
-const maxDelay = 10 * time.Second
-const minTimeDelta = 5 * time.Second
-const minProgressDelta = 0.05 // %
+const (
+	antiFlicker      = 5 * time.Second
+	maxDelay         = 10 * time.Second
+	minTimeDelta     = 5 * time.Second
+	minProgressDelta = 0.05 // %
+)
 
 type lastStatus struct {
 	Current   int64