GUAC-556: Send UPSTREAM_ERROR for unexpected disconnect and failure to connect in RDP.
GUAC-556: Add guac_client_abort() function for convenience.
GUAC-556: Send proper errors for SSH.
GUAC-556: Use guac_client_abort() for RDP errors.
GUAC-556: Use guac_client_abort() for VNC errors. Use appropriate error codes.
GUAC-556: Abort at higher level in RDP message wait function.