`@orderby` should throw an error if incorrect column syntax is used

[metanoid]

Using Query.jl in conjunction with DataFrames.jl I find that I often make silly mistakes with the syntax, such as:

df |>
      @orderby(:IssueDate) |>
      DataFrame

instead of

df |>
      @orderby(_.IssueDate) |>
      DataFrame

In such cases, Query.jl simply ignores the @orderby line, and does so silently.

I think that if the arguments to @orderby do not resolve to something in df that can be ordered, there should be a warning thrown, if not an error.

[davidanthoff]

Agreed! Or, even better, it could just work with @orderby(:IssueDate) as well :)