Improve logging when parsing invalid frames

[dcorbacho]

When the MQTT reader receives an invalid frame, i.e. an HTTP request, it crashes with function_clause and badmatch but sometimes it might not give enough info on the contents of the frame. Parsing errors should be captured and logged with the contents of the frame.

 exception exit: {function_clause,
                        [{rabbit_mqtt_frame,parse_utf,
                             [<<>>],
                             [{file,"src/rabbit_mqtt_frame.erl"},{line,145}]},

    exception exit: {{badmatch,
                         <<"T / HTTP/1.0\r\nAccept-Encoding: gzip\r\nAccept-Language: en\r\nAccept-Char">>},
                     [{rabbit_mqtt_frame,parse_frame,3,
                          [{file,"src/rabbit_mqtt_frame.erl"},{line,104}]},