handle graphviz parsing errors more cleanly

tests/functional_markdown_tests.py

@@ -27,3 +27,13 @@ def test_graphviz_is_rendered():
     assert response.status_code == 200
     assert b'~~~pydot' not in response.data
     assert b'data:image/png;base64' in response.data
+
+
+def test_invalid_graphviz_is_not_rendered():
+    """Check that invalid graphviz doesn't blow things up."""
+    app = app_with_pydot()
+    client = app.test_client()
+
+    response = client.get('/test-invalid-graphviz')
+    assert response.status_code == 500
+    assert b'INTERNAL SERVER ERROR' in response.data