def error(msg, line = nil)
      error = ::Opal::SyntaxError.new(msg)
      error.location = Opal::OpalBacktraceLocation.new(file, line)
      raise error
    end
def error(msg, line = nil)
      error = ::Opal::SyntaxError.new(msg)
      error.location = Opal::OpalBacktraceLocation.new(file, line)
      raise error
    end
This is called when a parsing/processing error occurs. This
 method simply appends the filename and curent line number onto
 the message and raises it.