Mono: Do not spam script class not found error

Print this error only when trying to instantiate the script. This way we prevent errors being printed for source files which are not meant to be used as scripts.

modules/mono/csharp_script.cpp

@@ -1954,8 +1954,12 @@ Variant CSharpScript::_new(const Variant **p_args, int p_argcount, Variant::Call
 
 ScriptInstance *CSharpScript::instance_create(Object *p_this) {
 
-	if (!valid)
-		return NULL;
+	if (!script_class) {
+		ERR_EXPLAIN("Cannot find class " + name + " for script " + get_path());
+		ERR_FAIL_V(NULL);
+	}
+
+	ERR_FAIL_COND_V(!valid, NULL);
 
 	if (!tool && !ScriptServer::is_scripting_enabled()) {
 #ifdef TOOLS_ENABLED
@@ -2045,20 +2049,15 @@ Error CSharpScript::reload(bool p_keep_state) {
 	if (project_assembly) {
 		script_class = project_assembly->get_object_derived_class(name);
 
-		if (!script_class) {
-			ERR_PRINTS("Cannot find class " + name + " for script " + get_path());
-		}
+		valid = script_class != NULL;
+
+		if (script_class) {
 #ifdef DEBUG_ENABLED
-		else if (OS::get_singleton()->is_stdout_verbose()) {
 			OS::get_singleton()->print(String("Found class " + script_class->get_namespace() + "." +
 											  script_class->get_name() + " for script " + get_path() + "\n")
 											   .utf8());
-		}
 #endif
 
-		valid = script_class != NULL;
-
-		if (script_class) {
 			tool = script_class->has_attribute(CACHED_CLASS(ToolAttribute));
 
 			native = GDMonoUtils::get_class_native_base(script_class);