Anyone who queries signatures, imports/exports keys, or who gains access to any of the encrypted keys (or indirectly?).  I have avoided propagating errors messages from crypto libraries, so it is in the cases where crypto operations are executed.

PyCrypto example:
https://github.com/dlitz/pycrypto/blob/master/lib/Crypto/PublicKey/RSA.py#L658-L660

I wouldn't intentionally print private keys, although the crypto library might if it encounters an unexpected exception.

Not sure if Zane is asking if the error string is too generic or requesting that we propagate exceptions from crypto libraries.