Document that non-Encryption SDK exceptions can be raised

The API documentation for the Python Encryption SDK lists [custom exception types](https://aws-encryption-sdk-python.readthedocs.io/en/latest/generated/aws_encryption_sdk.exceptions.html), but the APIs may let through exceptions from underlying libraries (e.g. `InvalidTag` and `InvalidSignature`) from pyca/cryptography. We should add a note to the documentation stating this.

Alternatively (or additionally) we can try to wrap these kind of exceptions into Encryption SDK custom exceptions with additional context.