[GRAPH-1099] Allows config error to return a map

[bryanjos]

This PR makes it so that if config/2 returns an error tuple where the second element is a map, then that map is treated as the error response.

Subscription config errors currently expect a tuple of {:error, msg }. If message is an map, when returned to the user, it looks like:

"errors": [
    {"message": {"extensions": {"code": "BAD_REQUEST"}, "message": "Invalid argument"}}
]

This causes trouble for us as we return spec compliant errors and they get wrapped in the message.

This PR makes it so returning the following from config/2

{:error, %{message: "Invalid argument", extensions: %{code: "BAD_REQUEST"}}

Will return

"errors": [
    {"extensions": {"code": "BAD_REQUEST"}, "message": "Invalid argument"}
]

[michaelcaterisano]

I think this api works, but I was hoping we could make it match a resolver's api, which supports returning either {:error, message} or {:error, spec_compliant_error_map}, i.e.
{:error, %{message: message, extensions: %{code: bad_request_code(), retry: false}}}

Was there a reason you decided against that api?

[bryanjos]

Yes because of what I explained about the spec_compliant_errors option. I figure it's the best way to still allow for the result phase to work how it does now with the least amount of unexpected consequences. I can do it differently and make changes in the result phase, but that feels weird to me.

And I figured it's the best way to get it into absinthe. I'm making an issue now and can change things depending on the feedback

[michaelcaterisano]

Ah I see Maarten's PR that added this, addressing this issue: absinthe-graphql/absinthe#925

Personally I think it's more straightforward to just return a map in the correct shape rather than have Absinthe format the errors for you. That's what we do now. IMO, the issue above isn't actually an issue, just a misunderstanding.

That said, if Absinthe is going to move forward with Maarten's fix (it's opt-in now, and to my knowledge we don't use it), then we should make sure it's well documented too.

[bryanjos]

Yeah I guess knowing the history of it now, it seems like it would make more sense to allow it as is