New assertion: isError

[SaltwaterC]

Hi,

I often find myself writing

assert.instanceOf(err, Error, 'some message about it')

It would be nice to have a shorter version for checking that an error is actually an Error, like:

assert.isError(err, 'some message about it')

Less keystrokes and better semantics.

[logicalparadox]

Please use plugin api.