Make divide-by-zero error, handle error gracefully in grow

grow/definition.go

@@ -52,11 +52,8 @@ func (def *Definition) Grow(statusChan chan<- Status) (*vm.Program, error) {
 
 		score, err := def.score(prog)
 		if err != nil {
-			if statusChan != nil {
-				close(statusChan)
-			}
-
-			return nil, err
+			// Can never get best score
+			continue
 		}
 
 		if score > status.BestScore {
@@ -89,7 +86,10 @@ func (def *Definition) score(prog *vm.Program) (uint64, error) {
 
 		sample.SetInputs(state)
 
-		state.Execute() // ignore error
+		err = state.Execute()
+		if err != nil {
+			return 0, err
+		}
 
 		score += sample.matchingOuts(state)
 	}