Fix validsig usage and error messages.
Fix validsig usage and error messages.

Reported by tphyahoo:

https://github.com/spesmilo/sx/issues/18

kaostao/sx,cancoin/libbitcoin-explorer,libmetrocoin/libmetrocoin-explorer,BWallet/sx,libmetrocoin/libmetrocoin-explorer,kaostao/sx,GroestlCoin/libgroestlcoin-explorer,cancoin/libbitcoin-explorer,GroestlCoin/libgroestlcoin-explorer,BWallet/sx,libtetcoin/libtetcoin-explorer,libtetcoin/libtetcoin-explorer,kaostao/sx,tuomassiren/libbitcoin-explorer,GroestlCoin/libgroestlcoin-explorer,BWallet/sx,tuomassiren/libbitcoin-explorer