Add failure message for cross client application
Add failure message for cross client application

`pyethapp`'s jsonrpc has a few non-spec methods. For `eth_nonce` we
catch the `Method not found` error and raise an Exception with a more
specific error message.

changwu-tw/pyethapp,gsalgado/pyethapp,ethereum/pyethapp,gsalgado/pyethapp,ethereum/pyethapp,changwu-tw/pyethapp