Review error handling

[cdegroot]

=CRASH REPORT==== 2-Feb-2017::22:19:06 ===
  crasher:
    initial call: Elixir.KafkaEx.Server0P9P0:init/1
    pid: <0.699.0>
    registered_name: 'cw:kafkanary:41'
    exception exit: {function_clause,
                        [{'Elixir.KafkaEx.Protocol.Fetch',parse_response,
                             [nil],
                             [{file,"lib/kafka_ex/protocol/fetch.ex"},
                              {line,49}]},```

Fetch errors now result in process crashes - and immediate restarts, and thus immediate exhaustion of supervisor restart maxima. Some very frequently-occurring errors probably should be handled.