* warden_server: Added "events_id" key into errors along with "events" - makes...

* warden_server: Added "events_id" key into errors along with "events" - makes error messages bigger, but allows client operators to identify offending messages by stable identifiers based on logs.
* Removed too much internal info from database errors.
* Removed format type checking from Draft4Validator, using only explicit schema regexps - Draft4Validator raised FormatError istead of ValidationError, rendering iter_errors unusable.
* warden_client: Implemented retries on server errors.
* Client now honours send_events_limit from server - too long list of events is split and sent in chunks.
* getInfo and sendEvents update local send_events_limit according to one sent by server in info or error message.
* Logging is now explicit, not authomatic in Error class, allowing to log only single time at top level methods.
* More sane default client name.
* Errors from server are now checked and ensure correct format.
* warden_filer: Ditched error and retry handling, warden_client now does for us.