add notification about crash on JSON encoding error: Unterminated string starting at https://github.com/frol/kombu/commit/397ab666dc31ef336bb7f97596a24ab912b3f24d
more information in crash notification https://github.com/frol/kombu/commit/2505db1e3fec4bf6107c4725aae39e99fee7c223

There was some strange and unreproducible error in production that was required this try/except, it's not necessary for this pull request, sorry.