move all exception declaration to their own file

exceptions.php

@@ -0,0 +1,49 @@
+<?php
+
+class WebException extends Exception
+{
+    private $excname;
+
+    /**
+     * @param string $name
+     * @param string $reason
+     * @param int $code
+     */
+    function __construct($name, $reason, $code)
+    {
+        $this->excname = $name;
+        parent::__construct($reason, $code);
+    }
+
+    public function getName()
+    {
+        return $this->excname;
+    }
+}
+
+class SqlException extends Exception
+{
+    private $query;
+
+    public function __construct($message = "Sql Error", $query = "")
+    {
+        $this->query = $query;
+        parent::__construct($message, 0);
+    }
+
+    public function getSqlError()
+    {
+        return $this->getMessage().' while executing: '.$this->query;
+    }
+}
+
+class BookNotFoundException extends WebException {
+    public function __construct($code) {
+        parent::__construct('BookNotFound', "The book with code $code was not found", -404);
+    }
+}
+
+/**
+ * Exception raised when an invalid attribute name is accessed
+ */
+class InvalidAttributeException extends Exception { }