feature #953 Improve exception messages for mount() (Damien Walsh, themainframe)

This PR was squashed before being merged into the 2.0.x-dev branch (closes #953).

Discussion
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Improve exception messages for mount()

Improve the exception message if the `connect` method of a `ControllerProviderInterface` returns an invalid type.

Previously a misleading message about the arguments to `mount()` was emitted if your `connect` method did not return a `ControllerCollection`.

Commits
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7d6298d8 Improve exception messages for mount()

src/Silex/Application.php

@@ -466,11 +466,17 @@ class Application extends Container implements HttpKernelInterface, TerminableIn
      * @param ControllerCollection|ControllerProviderInterface $controllers A ControllerCollection or a ControllerProviderInterface instance
      *
      * @return Application
+     *
+     * @throws \LogicException
      */
     public function mount($prefix, $controllers)
     {
         if ($controllers instanceof ControllerProviderInterface) {
             $controllers = $controllers->connect($this);
+
+            if (!$controllers instanceof ControllerCollection) {
+                throw new \LogicException('The "connect" method of the ControllerProviderInterface must return a ControllerCollection.');
+            }
         }
 
         if (!$controllers instanceof ControllerCollection) {