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/CConn.cxx b/vncviewer/CConn.cxx
index addc30d..2e97ec2 100644
--- a/vncviewer/CConn.cxx
+++ b/vncviewer/CConn.cxx
@@ -110,7 +110,8 @@
       vlog.info(_("connected to host %s port %d"), serverHost, serverPort);
     } catch (rdr::Exception& e) {
       vlog.error("%s", e.str());
-      fl_alert("%s", e.str());
+      if (alertOnFatalError)
+        fl_alert("%s", e.str());
       exit_vncviewer();
       return;
     }