Handle Error instances.

[KenjiTakahashi]

AFAICT they behave pretty much like regular objects, except that they have a tag of [object Error]. At the moment, they seem to fall through all the checks and just return false, without actually comparing anything.

[planttheidea]

Sure, I can add support for that tag. To-date they have been treated as reference equality, since there didn't seem to be a use-case to do even a shallow equality comparison, but easy enough to add.

[planttheidea]

It looks like standard object comparison doesn't work because the properties are not enumerable, however the properties on error instances are known and primitive values (much like regexp) so a custom comparator is easy enough.

[planttheidea]

Has been resolved as-of 5.1.0, as you can use the new areErrorsEqual method in a custom equality config.