Http has status for such conditions.
4XX
defines the error is from client side and needs a change.
Wiki says
The 4xx class of status code is intended for situations in which the client seems to have erred. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents should display any included entity to the user.[31]
For the condition where it is mentioned, its ideal to use 404 - Not Found
or 400 - Bad Request
This gives list of all the status codes and appropriate explanation.
W3Org
defined the specifications for these.