ssh: report disconnect messages as public error type from Conn.Wait

[bwhmather]

Replaces internal and external use of disconnectMsg as an error type for reporting back disconnects with a new public ssh.DisconnectError type.

This makes it possible to extract the disconnect reason code programmatically, and respond appropriately.

See golang/go#39259 for suggestion to export disconnect error message type.
Related golang/go#37913 for sending disconnect messages.

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Patch Set 1: Code-Review+1

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Patch Set 1: Hold+1

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