code: function logError (failure, index) {
    index = index + 1
  
    if (index > 1) {
      log('\n\n')
    }
  
    log('  ' + index + ') ' + failure.description + ': ')
  
    if (failure.environment) {
      log(('(' + failure.environment + ')').grey)
    }
  
    log('\n\n')
  
    // Log the actual error message / stack trace / diffs
    var intend = 4 + String(index).length
    var error = formatError(failure.error, intend)
  
    if (!_empty(error.message)) {
      log(error.message + '\n')
    }
  
    if (!_empty(error.stack)) {
      log('\n' + error.stack.grey + '\n')
    }
  }
docs: Log a single error