Server errors during the PWA generation ajax call are not handled

[mleroi]

If a server error occurs during PWA export, no feedback is displayed to the user for now.

[mleroi]

• in the sidbar export box, error feedback border's css class is wrong

[mleroi]

Fixed in https://github.com/uncatcrea/wp-appkit/commit/a2a4a3aa27c209a3a3a6aa0ce6f8494a5ab57143