Handle cases where the product is not found (for price addition)

[teolemon]

What

• Handle cases where the product is not found (for price addition)

Expected behavior

• Allow to add the price without adding the product
• Allow adding the product
• Allow doing both at the same time ? (would need an optional section in the product addition form)

Part of

• Open Prices (tracker) #5072

Screenshot/Mockup/Before-After

[monsieurtanuki]

Allow to add the price without adding the product

Done in #5392

[monsieurtanuki]

@teolemon A clean way would be to let the user enter the prices with unknown products (like now), then after clicking on "add the prices", instead of going back to the previous page, opening a new "hey you found new products, what about adding them to OFF?".