Don't leave errors on the stack in MdCtxRef::digest_verify_final

[alex]

No description provided.

[alex]

@sfackler let me know if you think this should just ignore teh errors, I'm not sure if Ok(false) ever actually occurs.

[botovq]

Ok(false) can happen: 1 signature is valid, 0 signature is invalid (typically no error on the stack), < 0 a library error occurred (typically error on the stack).

[sfackler]

This seems right to me. Must have been another example of a function where the return value docs used to be more specific.