catch and log payload handling errors

[nopolabs]

It doesn't look like you are maintaining slack-client any longer, but my bot receives killer channel_archive messages from Slack, I haven't caught the message itself, but here is the result:

PHP Fatal error: Uncaught ErrorException: Illegal string offset 'id' in /home/kip/php-kip/vendor/coderstephen/slack-client/src/RealTimeClient.php:406

This PR attempts to ameliorate by catching and logging errors thrown while handling payloads.

[sagebind]

I'd avoid Throwable if possible, since the library is currently targeting PHP 5.5+. We could boost it to PHP 7+ only as a new version, but we can't because there's some known compatibility issues between the websocket library being used and PHP 7.

[nopolabs]

I have changed the PR to use \Exception instead.
I am using a fork of devristo/phpws with updated react/socket and react/socket-client dependencies nopolabs/phpws@40c0be6 and haven't had any troubles.