Don't abort Node on unhandled rejections (#421)

[garybernhardt]

Fixes #421. Tests pass.

[lovasoa]

Thank you! We should add a test for the new behavior too.

[garybernhardt]

I spent a few minutes trying to come up with a test, but I can't get the bug to happen in the tests. I copied an existing test file and replaced its test function with:

exports.test = function(sql, assert) {
  // Intentionally leak a rejected promise
  Promise.reject("failure")
};

but that doesn't cause a test failure on the commit before the fix (e1ff5dc). I also tried various tricks to try to wait, in case the process was exiting too fast, but it seems like test functions can't return promises.