Extract model validation into a trait

Also use Laravel’s ValidationException rather than our own custom one

src/Core/Exceptions/ValidationFailureException.php

@@ -1,68 +0,0 @@
-<?php namespace Flarum\Core\Exceptions;
-
-use Exception;
-use Illuminate\Support\MessageBag;
-use DomainException;
-
-class ValidationFailureException extends DomainException
-{
-    /**
-     * @var MessageBag
-     */
-    protected $errors;
-
-    /**
-     * @var array
-     */
-    protected $input = array();
-
-    /**
-     * @param string $message
-     * @param int $code
-     * @param Exception $previous
-     */
-    public function __construct($message = '', $code = 0, Exception $previous = null)
-    {
-        parent::__construct($message, $code, $previous);
-
-        $this->errors = new MessageBag;
-    }
-
-    /**
-     * @param MessageBag $errors
-     * @return $this
-     */
-    public function setErrors(MessageBag $errors)
-    {
-        $this->errors = $errors;
-
-        return $this;
-    }
-
-    /**
-     * @return MessageBag
-     */
-    public function getErrors()
-    {
-        return $this->errors;
-    }
-
-    /**
-     * @param array $input
-     * @return $this
-     */
-    public function setInput(array $input)
-    {
-        $this->input = $input;
-
-        return $this;
-    }
-
-    /**
-     * @return array
-     */
-    public function getInput()
-    {
-        return $this->input;
-    }
-}