Fix EZP-21188: "Notice: Undefined variable: response" triggered when viewing a content with ezobjectrelationlist

Moved build of the response out of the try block in viewLocation() and viewContent(), to ensure $response is always correctly generated.
Furthermore got rid of Etag / LastModified response headers when viewing a content since it's useless / error prone because of client cache (aligned on cache of viewLocation).
Merge pull request #444 from ezsystems/ezp_21188

Fix EZP-21188: "Notice: Undefined variable: response" triggered when vie...