handle better and more ECDH Params
handle better and more ECDH Params

* properly raise a descriptive error when
* fix decode point to use proper opaque type (8 bits)

beni55/hs-tls,beni55/hs-tls,lancelotsix/hs-tls,lancelotsix/hs-tls,erikd/hs-tls,tolysz/hs-tls,erikd/hs-tls,tolysz/hs-tls