feat(http2): add new error variant for HTTP/2
feat(http2): add new error variant for HTTP/2

Automatic conversion from the `solicit::http::HttpError` is also
provided.

BREAKING CHANGE: A new variant `Http2` added to a public enum
`hyper::Error`.

hannobraun/hyper,bonifaido/hyper,hyperium/hyper,rschulman/hyper,golddranks/hyper,markuskobler/hyper,fhahn/hyper,hannobraun/hyper,fengsp/hyper,tshepang/hyper,golddranks/hyper,yazaddaruvala/hyper,Twey/hyper,giftcards/hyper,markuskobler/hyper,nickgonzales/hyper,yazaddaruvala/hyper,clatour/hyper,Twey/hyper,golddranks/hyper,nickgonzales/hyper,softprops/hyper,lame-nickname/hyper,hannobraun/hyper,steveklabnik/hyper,TimNN/hyper,bheesham/hyper,fuchsnj/hyper,nickgonzales/hyper,jmmaranan/hyper,yangxingpping/hyper,tempbottle/hyper,markuskobler/hyper,Keats/hyper,reem/hyper,tshepang/hyper,youprofit/hyper,erickt/hyper,achanda/hyper,sfackler/hyper,pyfisch/hyper,mre/hyper,Keats/hyper,mfeckie/hyper,ianks/hyper,manuel-woelker/hyper,hyperium/hyper,peterhj/hyper,mneumann/hyper,hyperium/hyper,Twey/hyper,Keats/hyper,tshepang/hyper,mitsuhiko/hyper