ErrorResponse can't be used as Rejection type

[jplatte]

Because ErrorResponse doesn't implement IntoResponse (it can't, instead Result<impl IntoResponse, ErrorResponse does), it can't be used as type Rejection of a FromRequest(Parts) implementation. I think we could fix this by removing the IntoResponse bound on type Rejection in the trait implementation and instead adding where Result<Self, Self::Rejection>: IntoResponse to the trait definition, but I'm also pretty sure that that bound then won't be implied by T: FromRequest and you'll have to drag it along everywhere you want to be generic over FromRequest.

Just wanted this documented, and might try it out again / find the relevant rustc issue.