fix(errors): SSH tunnel failures are not reported as a database authentication failure (#1305)

[ZhuchkaTriplesix]

Closes #1305

Problem

• SshAuthenticationException reads SSH authentication failed for user@host: ...; mapDatabaseError matched authentication failed, produced AuthFailedException (Check the username and password in the connection settings) and describeDatabaseError returned only that. A wrong bastion password looked like a wrong database password, and the bastion host, user and reason were lost.
• openTunnel wrapped every failure after the transport was open (a reset, a timeout, a protocol error) as SSH authentication failed; the jump host login and the transport connects threw raw driver errors.

Fix

• mapDatabaseError handles SshAuthenticationException, SshHostKeyMismatchException and the new SshConnectionException first, keeping the SSH text and adding a hint at the SSH tunnel settings (a host key mismatch: do not connect unless the server was reinstalled). A connection failure is HostUnreachableException.
• openTunnel (_sshFailure): SSHAuthError is an authentication failure; anything else is SshConnectionException with host and port. The transport connect (_dial) and the jump host login and forward are wrapped the same way, naming the jump host.
• describeDatabaseError keeps the hint of an unrecognised-but-hinted error.

Tests: tunnel — unreachable bastion names the host, a reset during the handshake is a connection failure, a rejected jump host login names jumper@jump.example; mapper — SSH auth is not AuthFailed and keeps the host, the one-line text points at the tunnel settings, a connection failure is unreachable, a mismatch says not to connect, a database password authentication failed is still AuthFailed.