`NestedValidationException` should include all `AbstractRelated` rule
exceptions.
`AbstractRelated` rule failures always indicate an interesting nested
context, which should not be omitted from the final result.
`NonOmissibleExceptionInterface` is a marker interface for
exceptions thrown by instances of `AbstractRelated`, which
facilitates identification of those rules' exceptions with
`instanceof`.
Many changes were made on `ValidationException::stringify`:
- Add support for instances of `Exception`;
- Add support for instances of `Traversable`;
- Add support for resources;
- Improve `Array` conversion;
- Improve `Object` conversion;
- Improve conversion of all values by using JSON.
Now, all the parameters of the exception classes are just converted to
string when replacing parameters on exceptions, so the exception classes
now keep the original value of all parameters.
