Support `code` expectation in t.throws()

[novemberborn]

#1704 introduces an expectation object for t.throws(). It'd be neat if we could add an expectation for the code property in Node.js.

We could consider supporting errno as well. I suspect code may be the more future proof property though, in which case I wonder if errno support is really needed.

[sindresorhus]

👍 to code, 👎to errno.