engine: graph: We shouldn't complain on context cancellation

These are expected from our engine. We do care about timeout's and so
on. This allows os to return ctx.Err() whenever a <-ctx.Done() happens,
which is more idiomatic for what we really want, but which we weren't
thorough with before.

engine/graph/actions.go

@@ -531,6 +531,9 @@ func (obj *Engine) Worker(vertex pgraph.Vertex) error {
 			if err == nil { // || err == engine.ErrClosed
 				return // exited cleanly, we're done
 			}
+			if err == context.Canceled {
+				return // we shutdown nicely on request
+			}
 			// we've got an error...
 			delay = res.MetaParams().Delay