general refactor, move more classes under Database namespace

src/Database/Exceptions/TenantDatabaseUserAlreadyExistsException.php

@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stancl\Tenancy\Database\Exceptions;
+
+use Stancl\Tenancy\Contracts\TenantCannotBeCreatedException;
+
+class TenantDatabaseUserAlreadyExistsException extends TenantCannotBeCreatedException
+{
+    public function __construct(
+        protected string $user,
+    ) {
+        parent::__construct();
+    }
+
+    public function reason(): string
+    {
+        return "Database user {$this->user} already exists.";
+    }
+}