crypto/tls: better error message when connecting to SSLv3 servers.
crypto/tls: better error message when connecting to SSLv3 servers.

We support SSLv3 as a server but not as a client (and we don't want to
support it as a client). This change fixes the error message when
connecting to an SSLv3 server since SSLv3 support on the server side
made mutualVersion accept SSLv3.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5545073

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