security-proxy - returns earlier in case of 403|404 error code (#506)

the Security-Proxy provides some specific JSPs for 403 and 404, but
before this commit, the proxified response for 403 / 404 were returned.

Forcing JSPs in case of 404 / 403 upstream responses.

Tested:
- locally
- on intra-test (Rennes Métropole)