Move exceptions to top-level namespace [BC break]

Currently, exceptions are in Grifart\Stateful\Exceptions, but are defined in files section of autoloading. This probably mismatch PSR-4 standard as reported here (relevant SA question).

I suggest to move exceptions to the top-level namespace so it corresponds to where the file is placed actually. We use this pattern in apps as well so there is no point in doing it other way here, I think.