security-proxy - bad HTTP response in case of downstream "connection refused"

[pmauduit]

The security-proxy currently returns a "200 - OK" status with a blank content in case of unavailable downstream tomcats.

I think a "502 - Bad gateway" or a "503 - Service unavailable" should be more relevant.

Step to "reproduce":

• Launch the proxycas tomcat
• shut down the georchestra tomcat
• attempt to connect onto http[s]://host/mapfishapp/

[fvanderbiest]

+1, this would maximize the benefit of having the ErrorDocument setup as per https://github.com/georchestra/georchestra/blob/14.12/doc/setup/apache.md#configuration

These error documents are currently shown (only) when the tomcat-proxycas is unavailable.