3.0 clean exceptions

* Moved all HTTP exceptions to the `Network` namespace
* Removed the usage of `Cake\Core\Exception` when a more appropriate exception could be used
* Removed the dependency from `ORM` to `Network` by re-implementing the exception
* Throwing more descriptive exceptions in Email, as `SocketException` always sounded to me as a network error.

Refs https://github.com/cakephp/cakephp/issues/4416
Thanks :+1: