define standard no-op keyring errors

Following on from #131, we need to define standard errors and error messages for the following events:

- On encrypt, this keyring is unwilling to create any decryption artifacts.
- On decrypt, this keyring is unwilling to fulfill its decryption contract.

Theses replace historical no-op behavior.

Read issue #131 and this issue's two listed encrypt and decrypt events first. Define the standard errors and error messages for both cases, replacing the historical no-op behavior; completion means both keyring behaviors have agreed, documented standards.

Written by the indexing model from the issue text.