Improve error messages for missing export presets when using `--export`

- Display a specific error message if the project doesn't have an
  `export_presets.cfg` file.
- Display a list of detected export presets if an invalid export
  preset name is supplied.

(cherry picked from commit 09f38ea2150cbf6aa3915bd2f5734df7ef24c88f)

editor/editor_node.cpp

@@ -706,17 +706,27 @@ void EditorNode::_fs_changed() {
 			}
 			preset.unref();
 		}
+
 		if (preset.is_null()) {
-			export_error = vformat(
-					"Invalid export preset name: %s. Make sure `export_presets.cfg` is present in the current directory.",
-					preset_name);
+			DirAccessRef da = DirAccess::create(DirAccess::ACCESS_RESOURCES);
+			if (da->file_exists("res://export_presets.cfg")) {
+				export_error = vformat(
+						"Invalid export preset name: %s.\nThe following presets were detected in this project's `export_presets.cfg`:\n\n",
+						preset_name);
+				for (int i = 0; i < EditorExport::get_singleton()->get_export_preset_count(); ++i) {
+					// Write the preset name between double quotes since it needs to be written between quotes on the command line if it contains spaces.
+					export_error += vformat("        \"%s\"\n", EditorExport::get_singleton()->get_export_preset(i)->get_name());
+				}
+			} else {
+				export_error = "This project doesn't have an `export_presets.cfg` file at its root.\nCreate an export preset from the \"Project > Export\" dialog and try again.";
+			}
 		} else {
 			Ref<EditorExportPlatform> platform = preset->get_platform();
 			const String export_path = export_defer.path.empty() ? preset->get_export_path() : export_defer.path;
 			if (export_path.empty()) {
-				export_error = vformat("Export preset '%s' doesn't have a default export path, and none was specified.", preset_name);
+				export_error = vformat("Export preset \"%s\" doesn't have a default export path, and none was specified.", preset_name);
 			} else if (platform.is_null()) {
-				export_error = vformat("Export preset '%s' doesn't have a matching platform.", preset_name);
+				export_error = vformat("Export preset \"%s\" doesn't have a matching platform.", preset_name);
 			} else {
 				Error err = OK;
 				if (export_defer.pack_only) { // Only export .pck or .zip data pack.
@@ -729,7 +739,7 @@ void EditorNode::_fs_changed() {
 					String config_error;
 					bool missing_templates;
 					if (!platform->can_export(preset, config_error, missing_templates)) {
-						ERR_PRINT(vformat("Cannot export project with preset '%s' due to configuration errors:\n%s", preset_name, config_error));
+						ERR_PRINT(vformat("Cannot export project with preset \"%s\" due to configuration errors:\n%s", preset_name, config_error));
 						err = missing_templates ? ERR_FILE_NOT_FOUND : ERR_UNCONFIGURED;
 					} else {
 						err = platform->export_project(preset, export_defer.debug, export_path);
@@ -739,13 +749,13 @@ void EditorNode::_fs_changed() {
 					case OK:
 						break;
 					case ERR_FILE_NOT_FOUND:
-						export_error = vformat("Project export failed for preset '%s', the export template appears to be missing.", preset_name);
+						export_error = vformat("Project export failed for preset \"%s\". The export template appears to be missing.", preset_name);
 						break;
 					case ERR_FILE_BAD_PATH:
-						export_error = vformat("Project export failed for preset '%s', the target path '%s' appears to be invalid.", preset_name, export_path);
+						export_error = vformat("Project export failed for preset \"%s\". The target path \"%s\" appears to be invalid.", preset_name, export_path);
 						break;
 					default:
-						export_error = vformat("Project export failed with error code %d for preset '%s'.", (int)err, preset_name);
+						export_error = vformat("Project export failed with error code %d for preset \"%s\".", (int)err, preset_name);
 						break;
 				}
 			}