add CustomError class

This will be used for throwing errors that can be caught and handled without having to parse the message.

src/utils/customError.test.ts

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2022 The Pybricks Authors
+
+import { CustomError } from './customError';
+
+// all subclasses should be declared like this:
+
+type TestErrorName = 'Test';
+
+class TestError extends CustomError<TestErrorName> {}
+
+describe('CustomError', () => {
+    it('should work with instanceof', () => {
+        expect(new TestError('Test', 'test')).toBeInstanceOf(TestError);
+    });
+
+    it('should override name', () => {
+        expect(new TestError('Test', 'test')).toHaveProperty('name', 'Test');
+    });
+
+    it('should pass cause', () => {
+        const testCause = new Error('test cause');
+        expect(() => {
+            throw new TestError('Test', 'test', testCause);
+        }).toThrow({ name: 'Test', message: 'test', cause: testCause });
+    });
+
+    it('should serialize to something useful', () => {
+        expect(JSON.stringify(new TestError('Test', 'test'))).toBe(
+            '"[TestError: Test]"',
+        );
+
+        expect(`${new TestError('Test', 'test')}`).toBe('[TestError: Test]');
+    });
+});

src/utils/customError.ts

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2022 The Pybricks Authors
+
+export class CustomError<T extends string> extends Error {
+    public constructor(name: T, message: string, cause?: Error) {
+        super(message, { cause });
+        this.name = name;
+        Object.setPrototypeOf(this, new.target.prototype);
+    }
+
+    public toString(): string {
+        return `[${this.constructor.name}: ${this.name}]`;
+    }
+
+    public toJSON(): string {
+        return this.toString();
+    }
+}