BUG/MEDIUM: http-ana: Don't loop trying to generate a malformed 500 response

BUG/MEDIUM: http-ana: Don't loop trying to generate a malformed 500 response

When HAProxy generates a 500 response, if the formatting failed, for instance
because the message is larger than a buffer, it retries to format it in loop. To
fix the bug, we must stop trying to send a response if it is a non-rewritable
response (TX_CONST_REPLY flag is set on the HTTP transaction).

Because this part is not trivial, some comments have been added.

No backport is needed.