Change where the HttpRequestDenied is returned

[rylev]

HttpRequestDenied errors are now returned async instead of immediately upon calling outgoing-handler/handle. I sort of think this is a worse behavior, and I do think we should figure out if it would be possible to keep the older behavior.

[lann]

After more consideration I think this may actually be better behavior. It seems pretty reasonable that any validation process that could result in HttpRequestDenied could require some asynchronous processing for a variety of reasons, which makes it an opportunity to do other concurrent work.

Really I think this applies to most of the error-codes, which suggests either:

• Assume that any error code can be returned from either handle or its future, equivalently
• Assume that (almost) all error codes will be returned exclusively from the future, with the possible exception of things like HTTP-request-URI-too-long