rework alerts

This starts moving alerts to the same subsystem where they are relevant
instead of putting everything in notifications.

So far, only the explorer file in use error is handled like this.

src/editor/error.ts

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2022 The Pybricks Authors
+
+import { CustomError } from '../utils/customError';
+
+/** Specific errors for editor subsystem. */
+export type EditorErrorName = 'FileInUse';
+
+/** Error class for editor subsystem. */
+export class EditorError extends CustomError<EditorErrorName> {}