Help text for Language domain needs to specify "no trailing slash"

This is Drupal core.

It should also validate for it IMHO.

Well, this no longer is allowed, but the following exception is thrown when adding a trailing slash:

Recoverable fatal error: Object of class Drupal\Core\Language\Language could not be converted to string in Drupal\Component\Utility\SafeMarkup::isSafe() (line 94 of core/lib/Drupal/Component/Utility/SafeMarkup.php).
Drupal\Component\Utility\SafeMarkup::isSafe(Object)
Drupal\Component\Utility\SafeMarkup::escape(Object)
Drupal\Component\Utility\SafeMarkup::placeholder(Object)
Drupal\Component\Utility\SafeMarkup::format('The domain for %language may only contain the domain name, not a protocol and/or port.', Array)
Drupal\Core\StringTranslation\TranslationManager->translate('The domain for %language may only contain the domain name, not a protocol and/or port.', Array, Array)
Drupal\Core\Form\FormBase->t('The domain for %language may only contain the domain name, not a protocol and/or port.', Array)
Drupal\language\Form\NegotiationUrlForm->validateForm(Array, Object)
call_user_func_array(Array, Array)
Drupal\Core\Form\FormValidator->executeValidateHandlers(Array, Object)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object, 'language_negotiation_configure_url_form')
Drupal\Core\Form\FormValidator->validateForm('language_negotiation_configure_url_form', Array, Object)
Drupal\Core\Form\FormBuilder->processForm('language_negotiation_configure_url_form', Array, Object)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\DependencyInjection\Container\prod\Symfony_Component_HttpKernel_HttpKernel_Proxy->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\DependencyInjection\Container\prod\Drupal_Core_StackMiddleware_Session_Proxy->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\Core\DependencyInjection\Container\prod\Drupal_Core_StackMiddleware_KernelPreHandle_Proxy->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)

This fixes the fatal error, and adds a mention of not using a trailing slash to the error message.

Tested patch and it worked as expected. Fatal error gone and text added to tell user not to use trailing slash.

Looks like we should have a test for this given Recoverable fatal error: Object of class Drupal\Core\Language\Language could not be converted to string.

I'll write a test.

This adds a test to ensure the form properly detects invalid domains.

Looks good to me.

This issue is a normal bug fix, and doesn't include any disruptive changes, so it is allowed per https://www.drupal.org/core/beta-changes. Committed c471f0f and pushed to 8.0.x. Thanks!