[RFE-5098] Support for customizing HAProxy error code response page for http error code 504

[natlibfi-jonollil]

I'm not sure if this is enough to make this work. Or should this be against master or next release version?

We have noticed problems regarding error response descriptions with HA-proxy. Problems have been encountered during timeouts and whitelisting as in these cases request produces blank response and this makes debugging work difficult. After finding out the reasons for the problems, we propose adding status codes 504 and 403 to the error responses to help identifying problems with timeouts and access rights in the future.

[openshift-ci]

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[openshift-ci]

Hi @natlibfi-jonollil. Thanks for your PR.

I'm waiting for a openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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