Fix assertions on functions that throw primitives

[RubenVerborgh]

Although not recommended, JavaScript functions can throw anything, i.e., not necessarily an Error.

However, Chai does not nicely deal with them. Consider the following snippet of code:

(function () { throw "test"; }).should.throw("test");
(function () { throw "test"; }).should.throw("whatever");
(function () { throw "test"; }).should.not.throw("whatever");

Currently, line 2 incorrectly doesn't cause an error, but line 3 does:

expected [Function] to not throw an error but 'test' was thrown

This pull request adds support for primitive errors, correctly resulting in this error for line 2:

expected [Function] to throw error including 'whatever' but got 'test'