JSON import: Validate structure of JSON file

Follow-up task from https://github.com/Automattic/simplenote-electron/pull/2446

See https://github.com/Automattic/simplenote-electron/pull/2446#discussion_r519026400

### Expected
Importing a JSON file that isn't a Simplenote export returns an errror.

### Observed
It crashes instead.

### Reproduced
1. Find a random .json file (not a Simplenote export)
2. Try to import it

Start by tracing the JSON import path described in the reproduction and compare it with the Simplenote export structure discussed in pull request #2446 and its linked review discussion. Done means a random non-Simplenote JSON file returns an error during import instead of crashing.

Written by the indexing model from the issue text.