GUI doesn't validate object name uniqueness accurately

Steps to reproduce:

1. On dashboard, click `+ Create New` to create a new maxscale object
2. Create a server with the same name as a monitor, e.g., if there is already a monitor
   name `Monitor`, now create a new server named 'Monitor.

Actual:
Receive error: "Can't create server 'Monitor', a monitor with that name already exists"

Expect:
A validation message should show to prevent the users from creating the object.