Improve error handling somewhat

- Fix composer crashing/not showing alert on error
- Make a general ValidationException which takes an array of field ⇒
messages to be outputted nicely by the API

src/Core/Exceptions/ValidationException.php

@@ -0,0 +1,18 @@
+<?php namespace Flarum\Core\Exceptions;
+
+use Exception;
+
+class ValidationException extends Exception
+{
+    protected $messages;
+
+    public function __construct(array $messages)
+    {
+        $this->messages = $messages;
+    }
+
+    public function getMessages()
+    {
+        return $this->messages;
+    }
+}