Fix reading of nested errros

Testing the objects with `instanceof static` will not work for
different classes that extend `Entity`, as `static` will refer to
the extending class, ie the objects would get tested against the
class on which the `errors()` method is being called.
Remove path assertion, rely on `testErrorPathReading`
Test against `EntityInterface` instead of `self`

Classes consuming `EntityTrait` do not necessarily have to
extend `Entity`, testing against `self` would fail in various
cases, like for example when one of the compared objects
extends `Entity` and the other just implements `EntityInterface`.
Merge pull request #5386 from ndm2/3.0-fix-reading-of-nested-errors

3.0 - Fix reading of nested errros