Add error handling

pre_commit_hooks/string_fixer_for_jupyter_notebooks.py

@@ -8,9 +8,17 @@ from pre_commit_hooks.util_string_fixer import fix_strings_in_file_contents
 
 
 def fix_strings(filename: str) -> int:
+    if not filename.endswith('.ipynb'):
+        print(f'{filename}: not a Jupyter notebook file')
+        return 1
+
+    try:
         with open(filename) as f:
             notebook_contents = json.load(f)
-
+    except json.JSONDecodeError as exc:
+        print(f'{filename}: Failed to load')
+        return 1
+    else:
         cells = notebook_contents['cells']
         return_value = 0
         for cell in cells: