'error.user_exists' => '該使用者登錄資訊已經存在。',
// TODO: translate the following.
// 'error.object_exists' => 'Object with this name already exists.',
-// 'error.client_exists' => 'Client with this name already exists.',
// 'error.invoice_exists' => 'Invoice with this number already exists.',
// 'error.role_exists' => 'Role with this rank already exists.',
// 'error.no_invoiceable_items' => 'There are no invoiceable items.',