improve error message

lib/server.js

@@ -1204,7 +1204,7 @@ WalletService.prototype.signTx = function(opts, cb) {
 
       if (!self.clientVersion || /^bw.-0\.0\./.test(self.clientVersion)) {
         if (!_.startsWith(txp.version, '1.')) {
-          return cb(new ClientError(Errors.codes.UPGRADE_NEEDED, 'This spend proposal was created using a newer version of the client app'));
+          return cb(new ClientError(Errors.codes.UPGRADE_NEEDED, 'To sign this spend proposal you need to upgrade your client app.'));
         }
       }
 

test/integration/server.js

@@ -4338,7 +4338,7 @@ describe('Wallet service', function() {
                 verifyStub.restore();
                 should.exist(err);
                 err.code.should.equal('UPGRADE_NEEDED');
-                err.message.should.contain('newer version');
+                err.message.should.contain('sign this spend proposal');
                 done();
               });
             });