- [Bug]: Enhanced log output when connecting to servers that do not
 support server-sig-algs extensions, making the new-as-of-2.9
 defaulting to SHA2 pubkey algorithms more obvious when it kicks in.

 - [Bug]: Connecting to servers which support server-sig-algs but
 which have no overlap between that list and what a Paramiko client
 supports, now raise an exception instead of defaulting to
 rsa-sha2-512 (since the use of server-sig-algs allows us to know
 what the server supports).

Signed-off-by: Javier Marcet <javier@marcet.info>