Add CryptoError info in keys.create_signature
Add CryptoError info in keys.create_signature

Add CryptoError info in securesystemslib.keys.create_signature()

Signed-off-by: Martin Vrachev <9a435b9cd6675a0f8cece6f0d3314da6092e8578@vmware.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib