Clean Up Exception Handling

[TimWolla]

The current handling Exceptions is historically grown and suffers from quite a few issues. Among these are unreadable god methods in core.functions.php, Layering Violations, because some Exceptions concern themselves with how they need to be rendered and duplicated logic splattered around in various places to work around short comings of this architecture.

The handling of exceptions should be cleaned up and turned into a clean pipeline where an exception comes in and then is acted upon by distinct handlers with a clear separation of concerns.

Specifically Exception should only transport information and not concern themselves with how they need to be logged or rendered into a HTTP response.

[TimWolla]

Moving to 5.6, the increased PSR-7 support might come in helpful.