Fix #1669: catch uncaught errors outside test suite execution

[danielstjules]

For #1669

$ cat test.js
setTimeout(function() {
  throw new Error('oops');
  it('test', function() {});
  run();
}, 100);

// Before PR
$ ~/git/mocha/bin/mocha --delay test.js
$ // EMPTY

// After PR
$ ~/git/mocha/bin/mocha --delay test.js

  1) Uncaught error outside test suite

  0 passing (5ms)
  1 failing

  1)  Uncaught error outside test suite:
     Uncaught Error: oops
      at null._onTimeout (test.js:2:9)

[danielstjules]

+1? @boneskull :)

[dasilvacontin]

Looks good! Thanks!

[danielstjules]

🎉 :)