crypto/ssh: clearer error messages when "no ciphers in common"

The error message reported by the ssh client when it can't find a
"cipher" in common between the client and server was overly vague.  This
adds more detailed error messages to findAgreedAlgorithms so that the
user can more easily identify which of the components can't reach
agreement.

Change-Id: I4d985e92fea964793213e5600b52b3141e712000
Reviewed-on: https://go-review.googlesource.com/13817
Reviewed-by: Adam Langley <agl@golang.org>