SCSSLint does not throw useful errors

[Paulus Schoutsen]

When an error occurs, we get KeyError: 'linter' instead of logging the reason the linter failed.

Error message the linter returns:

{
  '/tmp/tmpsyo2eogk/login.css': [{
    'reason': 'Syntax Error: Invalid CSS after "    filter: progid": expected ";", was ": DXImageTransf..."',
    'severity': 'error',
    'length': 1,
    'column': 1,
    'line': 248}]
}