Ensure we send 406 errors for unsupported accept types

[sentry-io]

Looks like we have bots etc poking around for things, not sure why this got logged into sentry.

For this class of situation we should gracefully return a 406.

Sentry Issue: LIGHTNING-AK

Phoenix.NotAcceptableError: no supported media type in accept header.

Expected one of ["html"] but got the following formats:

  * "application/dns-json" with extensions: []

To accept custom formats, register them under the :mime library
in your config/config.exs file:

    config :mime, :types, %{
      "application/xml" => ["xml"]
    }

And then run `mix deps.clean --build mime` to force it to be recompiled.

  File "lib/phoenix/controller.ex", line 1606, in Phoenix.Controller.refuse/3
  Module "Elixir.LightningWeb.Router", in LightningWeb.Router.browser/2
  File "lib/lightning_web/router.ex", line 1, in LightningWeb.Router.__pipe_through3__/1
  File "lib/phoenix/router.ex", line 475, in Phoenix.Router.__call__/5
  File "lib/lightning_web/endpoint.ex", line 1, in LightningWeb.Endpoint.plug_builder_call/2
...
(3 additional frame(s) were not displayed)

(Phoenix.NotAcceptableError no supported media type in accept header.

Expected one of ["html"] but got the following formats:

  * "application/dns-json" with extensions: []

To accept custom formats, register them under the :mime library
in your config/config.exs file:

    config :mime, :types, %{
      "application/xml" => ["xml"]
    }

And then run `mix deps.clean --build mime` to force it to be recompiled.
)