clean up and use consistent strategy for json parsing error handling in plugins

JSON parsing error handling is inconsistent in plugins (json and json_lines codecs, json fllter, gelf input, others?) in particular some log error, log warn or don't log at all.

I suggest that upon a json parser error, an **error** level log should be generated and the event should be tagged with `_jsonparsefailure`.

related to dicussion in GELF input PR #27