ESLint does not throw useful errors

[Paulus Schoutsen]

When the ESLint handler fails, it will currently raise a KeyError: 'ruleId' because the returned JSON does not match the expected output.

We should raise an appropriate error instead that contains the reason why ESLint failed to run.

Example output when an error occurs:

[
  {
    'errorCount': 1,
    'warningCount': 0,
    'filePath': '/tmp/tmpxsc5ni54/end_type_date.js',
    'messages': [
      {
        'fatal': True,
        'severity': 2,
        'column': 21,
        'message': 'Parsing error: Unexpected token =',
        'line': 23
      }
    ]
  }
]

[Paulus Schoutsen]

Fixed in #106