[feature] Check for and report bad protocol in TLSClientConfig.NextProtos

See issue #601.

When dialing a TLS connection, check for and return an error when Dialer.TLSClientConfig.NextProtos contains "h2". The error text should mention that this can happen when sharing a tls.Config with net/http Transport.

The problem in #601 is difficult for application developers to debug. An explicit error should help.

[ChannyClaus]

not sure if this is what you had in mind; https://github.com/gorilla/websocket/pull/788/files