Add support for verifymessage and better error reporting for sendrawtransaction

The first commit of this pull request contains an API implementation for the Bitcoin JSON RPC verifymessage call that can be used to verify a message signed with a Bitcoin address. The functionality can be accessed at `/api/messages/verify`, either via POST or GET, and it expects three parameters, `address`, `signature` and `message` either in query parameters or POST JSON data. It returns JSON in the form `{ result: [true|false] }`, or a 400 and message string on a bad request.

The second commit tries to make the error reporting of the sendRawTransaction API a bit more user friendly, so that it's easier to present in front-end implementations.

Both of these features, together with the matching front-end functionality, can be tested at http://koirikivi.fi:3001/ (links in the footer, next to the language selection). Note that the server runs against testnet, with both historic and p2p sync disabled (hence the warning on the index page), because of the limited hardware on the server.