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.

首先查看 issue #152，并跟踪 encrypt/decrypt 函数，以识别向用户暴露的常见错误。为选定的错误记录清晰的解释，并记录任何需要重新评估其清晰度的错误消息；完成的标准是 FAQ 涵盖已达成共识的错误情况，并且文案变更已经确定。

由索引模型根据 Issue 内容生成。