Rework our exception framework to fit into Kohana's model better.

Instead of overwriting Kohana_Exception::handle() (which we were doing
in MY_Kohana_Exception) we instead use their existing template system.

gallery/views/kohana/error.php overrides system/views/kohana/error.php
and is the standard error template for all exceptions.  Our version of
error.php figures out the appropriate view based on context (cli,
authenticated admin, guest viewing a 404, guest viewing a system
error) and delegates appropriately.  Each delegated view has a narrow
responsibility.

This paves the way for us to add new error views per module.  For
example, the rest module will define its own template in
Rest_Exception and then its exceptions can be rendered the way that it
wants (json encoded, in that case).

modules/gallery/views/error_cli.txt.php

@@ -0,0 +1,3 @@
+<?php defined("SYSPATH") or die("No direct script access.") ?>
+<?= Kohana_Exception::text($e) ?>
+