In TurboVNC, vncconnect works by writing an atom containing the remote connection info to the TurboVNC X server, and the TurboVNC Server reads that atom and initiates the reverse connection asynchronously. This does not provide an easy way of verifying whether the connection failed.
In TigerVNC and RealVNC, vncconnect utilizes additional functions in the VNC X11 extension. This allows it to operate synchronously and report an error if the connection fails. It should be straightforward to extend TurboVNC's implementation of the VNC extension to support similar functionality.