update structured output formats for human readable signature status

[David Bremner]

The current format includes the raw error number in the output. The
meaning of these numbers is documented only in the gmime
documentation, and changes between versions of gmime.

I'm not sure if this should include some code to fake the "valid" flag
provided by gmime 3.0; it would mean a bit more code duplication, but
might make the signature status more useful.