Add validation subscriber to check that Composer can be found.

Problem/Motivation

I can run the build tests locally but if I try to update I get this error

StatusText: 500 Service unavailable (with message)
ResponseText: The website encountered an unexpected error. Please try again later.PhpTuf\ComposerStager\Exception\ProcessFailedException: The "composer" executable cannot be found. Make sure it's installed and in the $PATH. in PhpTuf\ComposerStager\Domain\Stager->runCommand() (line 110 of vendor/php-tuf/composer-stager/src/Domain/Stager.php). PhpTuf\ComposerStager\Infrastructure\Process\Runner\AbstractRunner->findExecutable() (Line: 62)
PhpTuf\ComposerStager\Infrastructure\Process\Runner\AbstractRunner->run(Array, NULL, 120) (Line: 108)
PhpTuf\ComposerStager\Domain\Stager->runCommand(NULL, 120) (Line: 53)
PhpTuf\ComposerStager\Domain\Stager->stage(Array, '/Users/ted.bowman/sites/wdev/contrib_auto_updates_core_clone/.automatic_updates_stage') (Line: 298)
Drupal\automatic_updates\Updater->stageCommand(Array) (Line: 263)
Drupal\automatic_updates\Updater->stagePackages(Array) (Line: 251)
Drupal\automatic_updates\Updater->stageVersions(Array) (Line: 86)
Drupal\automatic_updates\BatchProcessor::stageProjectVersions(Array, Array) (Line: 295)
_batch_process() (Line: 137)
_batch_do() (Line: 93)
_batch_page(Object) (Line: 55)
Drupal\system\Controller\BatchController->batchPage(Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 578)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 716)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes