How to do an early stop? #87

Hi there 👋

I'm trying to implement a flag --limit-request-header-count on uvicorn, which I implemented changing the value of an attribute too_many_headers and raising an exception from on_header callback, see https://github.com/encode/uvicorn/pull/1683/files.

My question is: is my implementation as intended? Should the HttpParserCallbackError send more information when an exception happens from one of the callbacks?