eliot-prettyprint no longer dies on non-JSON lines

[itamarst]

Fixes #219.

[wallrj]

❓ Print these to stderr and include the failed line (or a snippet of)

[wallrj]

Thanks @itamarst

Looks good, but I think it should also skip non-eliot JSON lines. e.g. those logged by the docker daemon on journald systems.

4b7139d8-1ebc-4a07-a804-63fe0a536fee -> /1
2015-11-30 22:47:03.082205Z
  message_type: 'twisted:log'
  _HOSTNAME: '119.9.108.173'
  _SYSTEMD_UNIT: 'flocker-docker-plugin'
  error: False
  message: 'Stopping factory <twisted.web.client._HTTP11ClientFactory instance at 0x7f1b8da41518>'

Traceback (most recent call last):
  File "/home/richard/.virtualenvs/eliot-245/bin/eliot-prettyprint", line 9, in <module>
    load_entry_point('eliot==0.10.1+11.g61a813a', 'console_scripts', 'eliot-prettyprint')()
  File "/home/richard/projects/HybridLogic/eliot/eliot/prettyprint.py", line 103, in _main
    result = pretty_format(message) + "\n"
  File "/home/richard/projects/HybridLogic/eliot/eliot/prettyprint.py", line 67, in pretty_format
    level = "/" + "/".join(map(unicode, message[TASK_LEVEL_FIELD]))
KeyError: u'task_level'
(eliot-245)(not-json-219 ? =)[~/.../HybridLogic/eliot]$ egrep -A 1 4b7139d8-1ebc-4a07-a804-63fe0a536fee ../flocker/remote_logs.log 
{"_SYSTEMD_UNIT": "flocker-docker-plugin", "timestamp": 1448923623.082205, "task_uuid": "4b7139d8-1ebc-4a07-a804-63fe0a536fee", "_HOSTNAME": "119.9.108.173", "error": false, "message": "Stopping factory <twisted.web.client._HTTP11ClientFactory instance at 0x7f1b8da41518>", "message_type": "twisted:log", "task_level": [1]}
{"_SYSTEMD_UNIT": "docker", "message": "\u001b[34mINFO\u001b[0m[0000] API listen on [::]:2376                      \r", "_HOSTNAME": "119.9.108.173"}
(eliot-245)(not-json-219 ? =)[~/.../HybridLogic/eliot]$ 

Please merge when you've addressed that and added a test for that case.