ssh/tailssh: use a local error instead of gossh.ErrDenied (#10743)

ErrDenied was added in [our fork of
x/crypto/ssh](https://github.com/golang/crypto/commit/acc6f8fe8d618cba34d44e89fdde304f98c576df)
to short-circuit auth attempts once one fails.

In the case of our callbacks, this error is returned when SSH policy
check determines that a connection should not be allowed. Both
`NoClientAuthCallback` and `PublicKeyHandler` check the policy and will
fail anyway. The `fakePasswordHandler` returns true only if
`NoClientAuthCallback` succeeds the policy check, so it checks it
indirectly too.

The difference here is that a client might attempt all 2-3 auth methods
instead of just `none` but will fail to authenticate regardless.

Updates #8593

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

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