Clean up error handling in WalletKeyOpt functional options

[lrettig]

          Exiting the program in case the functional options have been defined in the wrong order is a bit extrem imo

I would prefer one of the following:

1. WithPdkdf2Password also takes a salt and the user must not also call WithSalt
2. WalletKey has a verify method that is called after applying all functional options that verifies a correct state and returns an error if something is missing (e.g. no salt provided)

Originally posted by @fasmat in #20 (comment)