assert verify_store_signature does not raise exception in case of internal crypto error

[bvavala]

The assert command here checks the verify_store_signature return value as a boolean value.
However, such return value originates from the signature verification API in the crypto lib, and it is a int -- the SWIG conversion should maintain the type. Most importantly, this value is -1 when there is an internal error in the crypto lib. In this case, the assert above does not raise any exception.
Also, this happens in other places.

Proposed solution: change the assert argument to (result > 0).

[cmickeyb]

result >= 0?

[bvavala]

No, I think it should be result > 0, because 0 means invalid and -1 means error

[bvavala]

Addressed in #486