[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
At this point, `$callable` is never a string like `MyController::fooAction` because `createController` has transformed it from that format into an actual object and method name, right? The first part of the `if` statement makes sense to me, but I'm not sure about the `strpos` part. I think I might just be missing how that's possible :).