Form-builder Preview not handling an unlikely error of loading invalid JSON

[thiessenp-cds]

It is possible for the Preview page to load invalid JSON and if this happens there is currently no good way to notify the user this happened.

For this error to occur, a user would need to directly access /form-builder/0000/preview and the parsed JSON from getSchema() contain invalid JSON. I've only been able to simulate this manually in the code. But it is still a possible error and potentially an error that would seriously confuse a user.

For example, a user would see this toast about invalid JSON. Then If s/he clicked download, the default template would be downloaded which is valid JSON. (and hide the file with invalid JSON)

An initial PR was done to handle thrown errors from JSON here: #3995

This is a followup PR to think about how best to handle this error case.