Improve t.throws output for mismatched constructor

[sholladay]

Description

The output for a failed t.throws() assertion should be clearer when the actual and expected constructors do not match.

Currently, it simply says that there was an unexpected exception and shows the error object. It is not immediately clear at first glance that the constructor is what caused the assertion to fail.

Test Source

const foo = async () => {
    throw new TypeError('Uh oh');
};

test('foo() does stuff', async (t) => {
    await t.throws(foo(), RangeError);
});

Output

  Threw unexpected exception:

  TypeError {
    message: 'Uh oh',
  }

Environment

Node 8.1.4
npm 5.3.0
ava 0.21.0
darwin 16.6.0

[sholladay]

A simple solution would be to replace this:

Threw unexpected exception:

... with something like:

Threw `TypeError` but expected `RangeError`:

[sindresorhus]

I was first thinking a diff:

  Threw unexpected exception:

- RangeError {
+ TypeError {
    message: 'Uh oh',
  }

But I think your suggestion would be better.

[novemberborn]

👍 on @sholladay's suggestion.

I suspect we still need to tackle #1047 (comment) first though.