Fix REST API

[raonismaneoto]

There are some troubles in the REST API that might confuse the users. Some of them are listed below, but the whole API must be checked for problems like these.

• The system returns 400 (Http Bad Request) when a GET request is done using an invalid Job Id. It should be 404 (Http Not found).
• The system returns 201 (Http Created) when a GET request is done using a valid Job Id. It should be 200 (Http OK).