[HttpKernel] Add better error message when controller action isn't callable

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | 

In the `ControllerResolver`, if a controller isn't callable, try to give a better description of what went wrong
This was done according to fabbot.io (http://fabbot.io/report/symfony/symfony/10788/16743fe4ac4b5929409b9f79e8349bb2a3460a9a, but the page doesn't exist anymore). I just applied the path, and assumed the changes is correct?