Added logic to check if a short code is in use and regenerate it otherwise

module/Core/src/Exception/ShortCodeCannotBeRegeneratedException.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Core\Exception;
+
+class ShortCodeCannotBeRegeneratedException extends RuntimeException
+{
+    /** @var @bool */
+    private $reasonIsSlug = false;
+
+    public static function forShortUrlWithCustomSlug(): self
+    {
+        $e = new self('The short code cannot be regenerated on ShortUrls where a custom slug was provided.');
+        $e->reasonIsSlug = true;
+
+        return $e;
+    }
+
+    public static function forShortUrlAlreadyPersisted(): self
+    {
+        return new self('The short code can be regenerated only on new ShortUrls which have not been persisted yet.');
+    }
+
+    public function reasonIsSlug(): bool
+    {
+        return $this->reasonIsSlug;
+    }
+}