lang: funcs: dage: Print out some error locations

Most things don't support this yet, but let's get in some initial
plumbing. It's always difficult to know which function failed, so we
need to start telling the users more precisely.

lang/funcs/dage/dage.go

@@ -1259,7 +1259,15 @@ func (obj *Engine) Run(ctx context.Context) (reterr error) {
 					obj.statsMutex.Unlock()
 				}
 				if runErr != nil {
-					obj.Logf("Erroring func `%s`: %+v", node, runErr)
+					err := fmt.Errorf("func `%s` errored: %+v", node, runErr)
+					displayer, ok := node.Func.(interfaces.TextDisplayer)
+					if ok {
+						if highlight := displayer.HighlightText(); highlight != "" {
+							obj.Logf("%s: %s", err.Error(), highlight)
+						}
+					}
+
+					obj.Logf("%s", err.Error())
 					// send to a aggregate channel
 					// the first to error will cause ag to
 					// shutdown, so make sure we can exit...