Allow extra headers in HTTPError

[florimondmanca]

Is your feature request related to a problem? Please describe.
We may want to send extra headers along with the response when an HTTP error occurs — particularly when dealing with authentication. We cannot do so currently without writing a custom error handler that would define the format of the response (plain text, JSON, …).

Describe the solution you'd like
Add a headers parameter to HTTPError, which users/clients can use to define extra headers to be sent with the response.

Describe alternatives you've considered

• Refactor HTTPError to be a subclass of Starlette's HTTPException (see Refactor HTTPError as a subclass of Starlette's HTTPException #274), and push this feature to the upstream Starlette project => incompatible with the philosophy of Starlette as an ASGI toolkit - see Add custom headers to HTTPException encode/starlette#507.

Implementation ideas
We'll need to update the base error handlers (error_to_text, errorto_json`, etc) to send the extra headers with the response.

Additional context
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[teaglebuilt]

is this feature still open?

[florimondmanca]

Hi @teaglebuilt,

Despite #334, yes it is (for now!). I would gladly review a PR to implement this piece of functionality. :)