Support for ordered sync/importing

- Order specified in config
- Tests for ordered sync
- New ErrorUpdatingModelException specifies which Model the error was thrown on

src/Exceptions/ErrorUpdatingModelException.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace distinctm\LaravelDataSync\Exceptions;
+
+use Exception;
+use Throwable;
+
+class ErrorUpdatingModelException extends Exception
+{
+    public function __construct(string $message = "", int $code = 0, Throwable $previous = null)
+    {
+        parent::__construct($message, $code, $previous);
+
+        $this->message = "Error updating the {$message} model.";
+    }
+}