add test checking if no error messages are left in notebook output (#19)

* add test checking if no error messages are left in notebook output. closes #12

* pylint and precommit fixes

test_notebooks.py

@@ -58,3 +58,15 @@ def test_run_notebooks(notebook_filename, tmp_path):
 def test_file_size(notebook_filename):
     """checks if all example Jupyter notebooks have file size less than a certain limit"""
     assert os.stat(notebook_filename).st_size * SI.byte < 1 * SI.megabyte
+
+
+def test_no_errors_or_warnings_in_output(notebook_filename):
+    """checks if all example Jupyter notebooks have clear std-err output
+    (i.e., no errors or warnings) visible"""
+    with open(notebook_filename, encoding="utf8") as notebook_file:
+        notebook = nbformat.read(notebook_file, nbformat.NO_CONVERT)
+        for cell in notebook.cells:
+            if cell.cell_type == "code":
+                for output in cell.outputs:
+                    if "name" in output and output["name"] == "stderr":
+                        raise AssertionError(output["text"])