Add option to escalate warning to error

Hello, I've a feature request here.

It would be useful to exit with status code ≠ 0 where currently there is just a warning.

For example, when decrypting, if the suppplied encryption context does not match that required there is only a `FAILED_VALIDATION` warning. With an error I can handle the different outcome based on the exit code.

I tried setting `PYTHONWARNINGS` to `error` but that only showed an error from ssl. I noticed nothing similar in the code.

Is this a feature you might implement?