Show full stack trace for non-assert errors

[vadimdemedes]

This PR fixes #271. Now AVA shows full stack trace for non-assertion errors. Before, it wrapped them in assert.AssertionError, which swallowed the original stack.

[sindresorhus]

It's pretty unclear what kind of error it is now, though:

var test = require('./');

test('wow', async t => {
    throw new Error('unicorn');
});

✖ wow unicorn

  1 test failed

  1. wow
  Error: unicorn
    at _callee$ (/Users/sindresorhus/dev/ava/example.js:4:8)
...

It just logs unicorn where you usually have true === false, etc.

We should somehow make it clear something in the test threw/rejected.

[vadimdemedes]

How about:

✖ wow failed with "unicorn"

[sindresorhus]

👍

[vadimdemedes]

Updated output:

[sindresorhus]

LGTM