Alert when create_hash_and_nullifier function is missing

[spalladino]

If a contract that creates notes does not implement this function, there is no error surfaced but the notes are just not there when queried. We should alert (emit a warning?) when processing notes that create_hash_and_nullifier is missing. If possible, we should also alert when building the contract, though this may be more difficult to do since we'd need to figure out if a contract is ever creating a note.

Edit: I was wrong on the above, there's currently an error being thrown, that breaks note processing. However, we should not stop processing notes due to the missing function, and log a warning instead.