get_output_cell fails with no such output

rather than returning null

IPython/html/tests/casperjs/util.js

@@ -73,7 +73,7 @@ casper.wait_for_output = function (cell_num) {
     });
 };
 
-// return the output of a given cell
+// return an output of a given cell
 casper.get_output_cell = function (cell_num, out_num) {
     out_num = out_num || 0;
     var result = casper.evaluate(function (c, o) {
@@ -81,7 +81,18 @@ casper.get_output_cell = function (cell_num, out_num) {
         return cell.output_area.outputs[o];
     },
     {c : cell_num, o : out_num});
-    return result;
+    if (!result) {
+        var num_outputs = casper.evaluate(function (c) {
+            var cell = IPython.notebook.get_cell(c);
+            return cell.output_area.outputs.length;
+        },
+        {c : cell_num});
+        this.test.assertTrue(false,
+            "Cell " + cell_num + " has no output #" + out_num + " (" + num_outputs + " total)"
+        );
+    } else {
+        return result;
+    }
 };
 
 // return the number of cells in the notebook