Check if downloaded pages are what they are supposed to be

[siphomateke]

Is your feature request related to a problem? Please describe.
Sometimes, the ZRA website bugs out and returns a 403 page instead of receipts. The extension then downloads those 403 error pages without showing any errors. The user thus thinks everything worked correctly even though it didn't.

Describe the solution you'd like
The extension should check if the downloaded pages are what they are supposed to be. E.g. if downloading a payment receipts, the correct fields should exist.