http: make error module public
http: make error module public

The `http::error::Error` type is re-exported from `http` as `HttpError`,
and while the `ErrorResponse` and `DiscordJsonError` types are public,
they aren't re-exported from the parent module and the `http::error`
module is private.

This means that the `Error::UnsuccessfulRequest` variant ends up not
being as useful as it could be, as seen here:
<https://docs.rs/serenity/0.8.0/serenity/http/enum.HttpError.html#variant.UnsuccessfulRequest>

Since these 3 types are the only ones in the `http::error` module, this
commit just makes the module public.

Signed-off-by: Vivian Hellyer <bcc12fd526d65050e68a2c2d18757b4e0d694e3a@hellyer.dev>

acdenisSK/serenity,Flat/serenity