Logging: change error to warning

This is in preparation for https://developer.blender.org/D8665.

source/blender/blenkernel/intern/anim_sys.c

@@ -2870,7 +2870,7 @@ void BKE_animsys_eval_driver(Depsgraph *depsgraph, ID *id, int driver_index, FCu
 
       /* set error-flag if evaluation failed */
       if (ok == 0) {
-        CLOG_ERROR(&LOG, "invalid driver - %s[%d]", fcu->rna_path, fcu->array_index);
+        CLOG_WARN(&LOG, "invalid driver - %s[%d]", fcu->rna_path, fcu->array_index);
         driver_orig->flag |= DRIVER_FLAG_INVALID;
       }
     }