Convert MonadError usages in Encryption into MonadThrow

Confcrypt.Encryption makes use of `throwError` from `Control.Monad.Except`, which can have unpredictable behavior when parallelizing calls. Instead, it makes sense to use the `MonadThrow` class and associated `Control.Exception` infrastructure because these natively support asynchronous exceptions.

Start in Confcrypt.Encryption by locating its throwError usages and the related Control.Monad.Except imports. Review MonadThrow and Control.Exception, then update the module so it uses the requested exception infrastructure; done means Encryption no longer relies on MonadError for these errors and supports asynchronous exceptions as described.

Written by the indexing model from the issue text.