-Properly handle missing ETC support on export

-Added ability for resource importers to save metadata
-Added ability for resource importers to validate depending on project settings

platform/javascript/export/export.cpp

@@ -167,6 +167,12 @@ bool EditorExportPlatformJavaScript::can_export(const Ref<EditorExportPreset> &p
 		}
 	}
 
+	String etc_error = test_etc2();
+	if (etc_error != String()) {
+		valid = false;
+		err += etc_error;
+	}
+
 	if (!err.empty())
 		r_error = err;