Make exceptions in event and command handling non-fatal

[dblock]

From https://github.com/dblock/slack-ruby-bot/issues/47

config.ru:20:in `block (2 levels) in <main>'
E, [2016-02-11T18:38:55.596707 #160] ERROR -- : listen loop error: undefined method `[]=' for nil:NilClass (NoMethodError)
E, [2016-02-11T18:38:55.596840 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/slack-ruby-client-0.6.0/lib/slack/real_time/event_handlers/pref_change.rb:9:in `call'
E, [2016-02-11T18:38:55.596881 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/slack-ruby-client-0.6.0/lib/slack/real_time/client.rb:161:in `run_handlers'
E, [2016-02-11T18:38:55.596909 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/slack-ruby-client-0.6.0/lib/slack/real_time/client.rb:155:in `dispatch'
E, [2016-02-11T18:38:55.596936 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/slack-ruby-client-0.6.0/lib/slack/real_time/client.rb:109:in `block (2 levels) in run_loop'
E, [2016-02-11T18:38:55.596960 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/event_emitter.rb:39:in `call'
E, [2016-02-11T18:38:55.596984 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/event_emitter.rb:39:in `block in emit'
E, [2016-02-11T18:38:55.597012 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/event_emitter.rb:38:in `each'
E, [2016-02-11T18:38:55.597061 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/event_emitter.rb:38:in `emit'
E, [2016-02-11T18:38:55.597108 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/faye-websocket-0.10.2/lib/faye/websocket/api/event_target.rb:45:in `dispatch_event'
E, [2016-02-11T18:38:55.597154 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/faye-websocket-0.10.2/lib/faye/websocket/api.rb:106:in `receive_message'
E, [2016-02-11T18:38:55.597202 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/faye-websocket-0.10.2/lib/faye/websocket/api.rb:40:in `block in initialize'
E, [2016-02-11T18:38:55.597253 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/event_emitter.rb:39:in `call'
E, [2016-02-11T18:38:55.597285 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/event_emitter.rb:39:in `block in emit'
E, [2016-02-11T18:38:55.597310 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/event_emitter.rb:38:in `each'
E, [2016-02-11T18:38:55.597334 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/event_emitter.rb:38:in `emit'
E, [2016-02-11T18:38:55.597378 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/hybi.rb:400:in `emit_message'
E, [2016-02-11T18:38:55.597427 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/hybi.rb:383:in `emit_frame'
E, [2016-02-11T18:38:55.597465 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/hybi.rb:121:in `parse'
E, [2016-02-11T18:38:55.597490 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/websocket-driver-0.6.3/lib/websocket/driver/client.rb:63:in `parse'
E, [2016-02-11T18:38:55.597530 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/faye-websocket-0.10.2/lib/faye/websocket/api.rb:145:in `parse'
E, [2016-02-11T18:38:55.597591 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/faye-websocket-0.10.2/lib/faye/websocket/client.rb:71:in `receive_data'
E, [2016-02-11T18:38:55.597641 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/eventmachine-1.0.9.1/lib/eventmachine.rb:193:in `run_machine'
E, [2016-02-11T18:38:55.597690 #160] ERROR -- : /app/vendor/bundle/ruby/2.2.0/gems/eventmachine-1.0.9.1/lib/eventmachine.rb:193:in `run'
E, [2016-02-11T18:38:55.597740 #160] ERROR -- : config.ru:20:in `block (2 levels) in <main>'
E, [2016-02-11T18:41:38.821515 #160] ERROR -- : Slack::Web::Api::Error: code_already_used