BUG: If a malformed .ipynb file exists in the root storage directory, the file browser fails to load

[mattprintz]

The user is therefore unable to see/fetch any files until the offending file is removed or fixed.

Beaker should skip malformed .ipynb files and treat them as plain text in the case that they cannot be parsed.

To recreate: save a notebook file and then manipulate the contents so that it is no longer valid json.