Changes

tsk71 Changes REST responses to HTTP status
Modifies REST resources to make the methods return always a response
with a HTTP status. Until now, only the ok responses were using this
kind of response, while the error responses were throwing Java
exceptions. Due to this, the tests are modified too to adapt them to
this new response system. Also a new ausiliary method was created to
check in the tests that a not found error is received.