http2: make CipherSuites validation error more verbose

Fixes golang/go#34776

Change-Id: Ib76e687780781d9cc59346f722d0ea4ec033ce77
Reviewed-on: https://go-review.googlesource.com/c/net/+/200317
Reviewed-by: Bryan C. Mills <bcmills@google.com>

http2/server.go

@@ -252,7 +252,7 @@ func ConfigureServer(s *http.Server, conf *Server) error {
 			}
 		}
 		if !haveRequired {
-			return fmt.Errorf("http2: TLSConfig.CipherSuites is missing an HTTP/2-required AES_128_GCM_SHA256 cipher.")
+			return fmt.Errorf("http2: TLSConfig.CipherSuites is missing an HTTP/2-required AES_128_GCM_SHA256 cipher (need at least one of TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 or TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256).")
 		}
 	}