[FrameworkBundle] A controller must be callable

Am I right to say that a controller **must** be callable, ie [method_exists()](https://github.com/vicb/symfony/pull/new/symfony:2.0...resolver#L1L77) is a bug ?

No BC, tests pass.

The change is a one liner however I have rewritten some code & tests to the current standards (what a long way since the genesis of Sf2 !)

I have also added a FIXME for Sf3, because returning `false` is too much `PHP` !