files.OfflineError prettier than [object Object]

(It should also be caught at every httpHelpers.getUrl call and handled
properly, but this is a start.)

tools/files.js

@@ -834,6 +834,9 @@ files.FancySyntaxError = function () {};
 files.OfflineError = function (error) {
   this.error = error;
 };
+files.OfflineError.prototype.toString = function () {
+  return "[Offline: " + this.error.toString() + "]";
+};
 
 // Like fs.readdirSync, but skips entries whose names begin with dots, and
 // converts ENOENT to [].