cmd/{dirlist,dnslist}: Clearer error message

when not choosing small/medium/large

cmd/dirlist.go

@@ -51,6 +51,9 @@ func Dirlist(size string, url string, timeout time.Duration, threads int, logdir
 		list = directoryURL + mediumFile
 	case "large":
 		list = directoryURL + bigFile
+	default:
+		log.Errorf("Invalid dirlist size %s. Please choose from small, medium, or large", portstyle.Render(size))
+		return
 	}
 
 	dirlog.Infof("Starting %s directory listing", size)

cmd/dnslist.go

@@ -39,6 +39,9 @@ func Dnslist(size string, url string, timeout time.Duration, threads int, logdir
 		list = dnsURL + dnsMediumFile
 	case "large":
 		list = dnsURL + dnsBigFile
+	default:
+		log.Errorf("Invalid dnslist size %s. Please choose from small, medium, or large", portstyle.Render(size))
+		return
 	}
 
 	dnslog.Infof("Starting %s DNS listing", size)