Display warning for unsupported data types

This fixes #50.

doc/code-cells.ipynb

@@ -344,6 +344,30 @@
     "YouTubeVideo('WAikxUGbomY')"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Unsupported Output Types\n",
+    "\n",
+    "If a code cell produces data with an unsupported MIME type, the Jupyter Notebook doesn't generate any output.\n",
+    "`nbsphinx`, however, shows a warning message."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "display({\n",
+    "    'text/x-python': 'print(\"Hello, world!\")',\n",
+    "    'text/x-haskell': 'main = putStrLn \"Hello, world!\"',\n",
+    "}, raw=True)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},

nbsphinx.py

@@ -159,7 +159,7 @@
         \\end{OriginalVerbatim}
 {% else %}
 
-    WARNING! Data type not implemented: {{ datatype }}
+    .. nbwarning:: Data type cannot be displayed: {{ datatype }}
 {%- endif %}
 {% endmacro %}
 
@@ -797,10 +797,14 @@ def _get_output_type(output):
             if datatype in output.data:
                 html_datatype = datatype
                 break
+        else:
+            html_datatype = ', '.join(output.data.keys())
         for datatype in DISPLAY_DATA_PRIORITY_LATEX:
             if datatype in output.data:
                 latex_datatype = datatype
                 break
+        else:
+            latex_datatype = ', '.join(output.data.keys())
     return html_datatype, latex_datatype