Create FAQ for common errors on encrypt/decrypt

Errors can bubble up from the encrypt/decrypt functions.
It would be nice to have clear explanations for common errors.

In creating this documentation,
the error messages can also be re-evaluated for clarity.

See #152 as an example.

Start by reviewing issue #152 and tracing the encrypt/decrypt functions to identify the common errors exposed to users. Document clear explanations for the selected errors, and record any error messages that need re-evaluation for clarity; done means the FAQ covers the agreed error cases and wording changes are decided.

Written by the indexing model from the issue text.