Report when the .nsprc file is present but not (valid) JSON

[alert-debug]

While testing the latest version, I made a small mistake when updating the .nsprc file which meant the file was no longer valid JSON. It only took me a moment to realise why none of the exclusions were being applied, but a beginner might be more confused (especially if they are using an editor which doesn't warn them about JSON validity issues in their files).

Hopefully it's a small change to catch the specific case of an invalid JSON file, and output a specific human-readable error message to the user.