Saving a class with no price now works, and a class that cannot be created tells you instead of failing quietly

Two fixes for creating and saving classes in your admin.

• A class saved without a price is now treated as free and saves normally, instead of the save failing when the price field was left untouched or cleared.
• If a class cannot be created, you now see a clear message straight away, instead of the class appearing to be created with its passes and rooms linked to nothing.