[kziptest] Print errorString() for easier debugging
Forwards error message from backend (e.g. "unrecognized header found") rather than just printing a generic "archive is invalid" error
Testing with an archive from Bug 415221 I now get ZIP file doesn't exist or is invalid: "Invalid ZIP file. Unrecognized header at offset 12312"
Unfortunately trying to open a non-existant archive gives me ZIP file doesn't exist or is invalid: "Could not set device mode to 1"`