The indentation is off here: it makes this check and prints the error on every iteration of the loop after the condition is true.

It might be more useful if we mentioned _which_ notebooks had the errors. E.g. something like this:

```
for nbook in unmerged_notebooks:
    except nbformat.NotJSONError:
        invalid_notebooks.append(filename)
# ...
if len(invalid_notebooks) > 0:
    print ('There was a problem parsing the following notebook files:\n' + '\n'.join(invalid_notebooks))