Add error messages to the RangeErrors

notebook/static/base/js/utils.js

@@ -239,13 +239,13 @@ define([
             g = numbers.shift();
             b = numbers.shift();
             if ([r, g, b].some(function (c) { return c < 0 || 255 < c; })) {
-                throw new RangeError();
+                throw new RangeError("Invalid range for RGB colors");
             }
         } else if (n === 5 && numbers.length >= 1) {
             // 256 colors
             var idx = numbers.shift();
             if (idx < 0) {
-                throw new RangeError();
+                throw new RangeError("Color index must be >= 0");
             } else if (idx < 16) {
                 // 16 default terminal colors
                 return idx;
@@ -261,10 +261,10 @@ define([
                 // grayscale, see http://stackoverflow.com/a/27165165/500098
                 r = g = b = (idx - 232) * 10 + 8;
             } else {
-                throw new RangeError();
+                throw new RangeError("Color index must be < 256");
             }
         } else {
-            throw new RangeError();
+            throw new RangeError("Invalid extended color specification");
         }
         return [r, g, b];
     }