Be strict about requiring ServerKeyExchange.

Missing ServerKeyExchange is handled, but only because it hits an
ERR_R_INTERNAL_ERROR in ssl3_send_client_key_exchange in trying to find the
server ECDH parameters. Be strict about requiring it for ECDHE.

Change-Id: Ifce5b73c8bd14746b8a2185f479d550e9e3f84df
Reviewed-on: https://boringssl-review.googlesource.com/1157
Reviewed-by: Adam Langley <agl@google.com>