core: improve message for assertion errors (throw assertion)

[andreineculau]

# create CustomError with prototype Error
fun = () -> throw new CustomError('test', true)
fun.should.not.Throw()

the above ends up showing an error message which is not useful at all (due to the Error instance being passed all the way to objDisplay) e.g.

AssertionError: expected [Function] to not throw an error but { Object (expected, found, ...) } was thrown

This PR, changes the message to e.g.

AssertionError: expected [Function] to not throw an error but 'CustomError: Expected x but found y' was thrown

[logicalparadox]

I like the change but the travis build failed. Can you please check the logs and refactor any tests so this passes for all environments.

[coveralls]

Coverage remained the same when pulling e174212 on andreineculau:patch-1 into 4b51ea7 on chaijs:master.

[andreineculau]

Sorry, I wasn't thinking straight.
I updated all relevant code, and the necessary tests.

[domenic]

From what I can tell of the tests, this change just replaces square brackets with single quotation marks. Why is that a good thing?

[logicalparadox]

Thanks! Worked great.