provide an error message

git-svn-id: https://nagiosplug.svn.sourceforge.net/svnroot/nagiosplug/nagiosplug/trunk@757 f882894a-f735-0410-b71e-b25c423dba1c

plugins/check_nt.c

@@ -316,7 +316,7 @@ int main(int argc, char **argv){
 
 	case CHECK_NONE:
 	default:
-		usage (_(""));
+		usage (_("Please specify a variable to check"));
 		break;
 
 	}