servers: fix '/connections/:id' error response

* Ensure error response is formatted as {error: message: 'xxxx'}.

backend/routes.js

@@ -457,11 +457,11 @@ export default class Servers {
                     return next();
                 }
                 Logger.log(validation, 2);
-                res.json(400, {error: validation.message});
+                res.json(400, {error: {message: validation.message}});
                 return next();
             }).catch(err => {
                 Logger.log(err, 2);
-                res.json(400, {error: err.message});
+                res.json(400, {error: {message: err.message}});
                 return next();
             });
         });

test/backend/routes.spec.js

@@ -1578,7 +1578,7 @@ describe('Routes:', () => {
                 .then(getResponseJson).then(json => {
                     assert.deepEqual(
                         json,
-                        {error: 'password authentication failed for user "banana"'}
+                        {error: {message: 'password authentication failed for user "banana"'}}
                     );
                     assert.deepEqual(
                         getConnections(),