Handle error as string

[vitalets]

When tap-mocha-reporter is used with native Node.js test-runner output, it receives parsed test result as follows (note error field):

{
  ok: false,
  id: 1,
  name: 'should work',
  diag: {
    duration_ms: 2.382542,
    failureType: 'testCodeFailure',
    error: 'Expected values to be strictly equal:\n\n1 !== 2', // <-- error is string not object!
    code: 'ERR_ASSERTION',
    expected: 2,
    actual: 1,
    operator: 'strictEqual',
    stack: 'TestContext.<anonymous> (file:///Users/vitalets/projects/myproject/test/calc.test.ts:9:10)\n' +
      'Test.runInAsyncScope (node:async_hooks:204:9)\n' +
      'Test.run (node:internal/test_runner/test:543:25)\n' +
      'Test.start (node:internal/test_runner/test:464:17)\n' +
      'test (node:internal/test_runner/harness:129:18)\n' +
      'file:///Users/vitalets/projects/myproject/test/calc.test.ts:5:1\n' +
      'ModuleJob.run (node:internal/modules/esm/module_job:194:25)'
  },
  fullname: '/Users/vitalets/projects/myproject/test/calc.test.ts'
}

But reporter expects error to be an object not string. That's why output does not contain error message and stack (see #72).
This PR fixes the issue by taking error message and stack from diag object directly.

[wesleytodd]

I haven't tested this, but @isaacs if this is something you could merge it would be really great. Other than this, I am having great success with the node test runner.

[isaacs]

This module is due for a major refactoring to pull these report styles into tap v18 as a plugin. I'll try to work through the backlog a bit today and tomorrow.