Better error message if gnuplot comm hangs

It no longer asks the user to complain to me. I'm fairly confident the comm
logic is sound now, so complaining to me is unhelpful to anybody. The most
common culprit is the gnuplot qt terminal, which hangs if you try to plot
without a DISPLAY set

Author: dkogan

gnuplotlib.py

@@ -1721,8 +1721,7 @@ def _checkpoint(self, flags=''):
                 self.checkpoint_stuck = True
 
                 raise GnuplotlibError(
-                    r'''Gnuplot process no longer responding. This is likely a bug in gnuplotlib
-and/or gnuplot itself. Please report this as a gnuplotlib bug''')
+                    r'''Gnuplot process no longer responding. This shouldn't happen... Is your X connection working?''')
 
         fromerr = re.search(r'\s*(.*?)\s*{}$'.format(checkpoint), fromerr, re.M + re.S).group(1)