fixes #6820 - apipie-bindings should enforce required params, BZ 1116803

- [x] validation of nested routes
- [x] do not rely on `gettext`
- [ ] tests!
@komidore64 I discussed that with Google and just about every article or blogpost suggested to inherit custom exceptions from StandardError. In my understanding RuntimeError is used rather for generic unhendled unexpected situation. So if you know what had happened and have specific exception for that raise it, Raise RuntimeError otherwise.