Add better consistency check and error output to plot script.

tools/profviz/stdio.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 26 matching lines @@
 // Convert picoseconds to milliseconds.
 distortion_per_entry = distortion / 1000000;
 var rangelimits = result.range.split(",");
 var range_start = parseInt(rangelimits[0]);
 var range_end = parseInt(rangelimits[1]);
 if (!isNaN(range_start)) range_start_override = range_start;
 if (!isNaN(range_end)) range_end_override = range_end;
 
 var kResX = 1600;
 var kResY = 600;
-var psc = new PlotScriptComposer(kResX, kResY);
+function log_error(text) {
+  print(text);
+  quit(1);
+}
+var psc = new PlotScriptComposer(kResX, kResY, log_error);
 psc.collectData(readline, distortion_per_entry);
 psc.findPlotRange(range_start_override, range_end_override);
 print("set terminal pngcairo size " + kResX + "," + kResY +
       " enhanced font 'Helvetica,10'");
 psc.assembleOutput(print);