Added handling for missing container/router in AppFactory

Unable to test, because both classes are always present in our test
suite!

- One tests for `$container` absence, and lack of `ServiceManager`
- One tests for `$router` absence, and lack of `Router\FastRouteRouter`

src/AppFactory.php

@@ -40,11 +40,31 @@ final class AppFactory
      * @param null|Router\RouterInterface $router Router implementation to use;
      *     defaults to the FastRoute router bridge.
      * @return Application
+     * @throws Exception\MissingDependencyException if the container was not
+     *     provided and the ServiceManager class is not present.
+     * @throws Exception\MissingDependencyException if the router was not
+     *     provided and the Router\FastRouteRouter class is not present.
      */
     public static function create(
         ContainerInterface $container = null,
         Router\RouterInterface $router = null
     ) {
+        if (! $container && ! class_exists(ServiceManager::class)) {
+            throw new Exception\MissingDependencyException(sprintf(
+                '%s requires a container, but none was provided and %s is not installed',
+                __CLASS__,
+                ServiceManager::class
+            ));
+        }
+
+        if (! $router && ! class_exists(Router\FastRouteRouter::class)) {
+            throw new Exception\MissingDependencyException(sprintf(
+                '%s requires a router, but none was provided and %s is not installed',
+                __CLASS__,
+                Router\FastRouteRouter::class
+            ));
+        }
+
         $container = $container ?: new ServiceManager();
         $router    = $router    ?: new Router\FastRouteRouter();
         $emitter   = new Emitter\EmitterStack();

src/Exception/MissingDependencyException.php

@@ -0,0 +1,14 @@
+<?php
+/**
+ * @see       https://github.com/zendframework/zend-expressive for the canonical source repository
+ * @copyright Copyright (c) 2016 Zend Technologies USA Inc. (http://www.zend.com)
+ * @license   https://github.com/zendframework/zend-expressive/blob/master/LICENSE.md New BSD License
+ */
+
+namespace Zend\Expressive\Exception;
+
+use RuntimeException;
+
+class MissingDependencyException extends RuntimeException implements ExceptionInterface
+{
+}