Improve exception message for Jason.Encoder errors

[ivanov]

Thanks for all that you do, including continued maintenance of ecto, and now taking the time to consider this small pull request.

I decided it was too much to replicate the full contextual block that was already there just to add one alternative @derive... line, and thought maybe a commented out approach would be both simple and useful.

[warmwaffles]

One alternative here is to show two separate examples without having a directive to uncomment a line.

    @doc """


      Or choose to not encode the association when converting the struct \
      to JSON by explicitly listing the JSON fields in your schema:

          defmodule #{inspect(owner)} do
            # ...
            @derive {Jason.Encoder, only: [:name, :title, ...]}
            schema ... do

      Or choose to not encode the association by excluding it in your schema:

          defmodule #{inspect(owner)} do
            # ...
            @derive {Jason.Encoder, except: [#{inspect(field)}]}
            schema ... do

 
    """

I know that when I am looking at docs, I usually gravitate to the examples without paying attention to the helpful text above it 💀.

[ivanov]

I'm happy to do convert to that style, if that's preferred (also happy if maintainers want to adjust this in that direction). The only thing to add is that this isn't part of narrative documentation, this is traceback you will get if your code causes this error, so that's why I was trying to lean towards being more terse.

[josevalim]

I believe except is anti-pattern because it is easy to leak information you don't want but it is ok to mention it. :)

[josevalim]

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