fixed missing test

exercises/matrices/matrices-solution.ipynb

@@ -1196,7 +1196,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 27,
+   "execution_count": 41,
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1221,7 +1221,16 @@
     "        ['g','h','i']\n",
     "     ]\n",
     "\n",
-    "assert diag(m) == ['a','e','i']\n"
+    "assert diag(m) == ['a','e','i']\n",
+    "\n",
+    "try: \n",
+    "    diag([['a','b']])\n",
+    "    raise Exception(\"SHOULD HAVE FAILED !\")  # if diag raises an exception which is ValueError as we expect it to do,\n",
+    "                                             # the code should never arrive here\n",
+    "except ValueError: # this only catches ValueError. Other types of errors are not catched\n",
+    "    \"passed test\"  # In an except clause you always need to put some code. \n",
+    "                   # Here we put a placeholder string just to fill in\n",
+    "\n"
    ]
   },
   {