Dedicated exception classes for various failure scenarios

[willemstuursma]

It would be great to have dedicated exceptions for various failure scenarios. Now there is only DecodingFailedException. Other failure scenarios that could use their own exception class:

• SignatureValidationFailedException - the signature is not valid or was not created using Apple's private key
• PaymentDataExpiredException - the payment data has expired
• DecryptingFailureException - the data cannot be decrypted using the private key provided.

Now, we have to resort to substring matching on the exception message.

[trydocatch]

Hi @willemstuursma ,

That is a good idea. I'll try to see if there are any cases when it would be helpfull to throw a dedicated exception