Allow removal of GUI prompt on fatal errors

Add a new parameter 'alertOnFatalError' which guards
the displaying of the GUI alert on fatal errors, and
thus when false just gives the textual error.

Now I can do:

  while true
  do
    vncviewer alertOnFatalError=false vm:0
    sleep 1
  done

and it'll reappear when my VM appears without me getting error
dialogs.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
--
diff --git a/vncviewer/vncviewer.man b/vncviewer/vncviewer.man
index b1450d1..e48a0c4 100644
--- a/vncviewer/vncviewer.man
+++ b/vncviewer/vncviewer.man
@@ -275,6 +275,10 @@
 \fIR\fR, and \fIG\fR taken the values of the local port number, the remote
 host, the port number on the remote host, and the gateway machine
 respectively.
+.
+.TP
+.B \-AlertOnFatalError
+Display a dialog with any fatal error before exiting. Default is on.
 
 .SH FILES
 .TP