Enforce throwing error object with t.throws

[sindresorhus]

Supporting throwing string (which is a very bad practise anyways) leads to problems like #1439.

import test from 'ava';

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

  1 failed

  [anonymous]
  /Users/sindresorhus/dev/private/ava-playground/test.js:4

   3: test(t => {
   4:   t.throws(() => {
   5:     throw 'foo';

  Threw unexpected exception:

  'foo'

It's not entirely clear here that the problem is that it's excepting an Error object, but a string was thrown. Instead of improving the error message for this case, we should just disallow throwing a string altogether, so when a string is thrown, we could warn like this:

Threw a string, but the assertion requires an Error object