t.throws not that helpful when the message assertion does not match

[jamestalmage]

test(t => {
    t.throws(() => {
        throw new Error('foo');
    }, 'bar');
});

output:

   1 failed

  1. [anonymous]
  foo
      Test.fn (test.js:61:4)

It should show both the expected and actual message. (currently just shows the actual message foo).

[novemberborn]

We should implement t.throws ourselves, rather than wrapping assert.throws.

[novemberborn]

Closing in favor of #1047. See also #661.