BUG/MINOR: backend: Fallback on RR algo if balance on source is impossible

BUG/MINOR: backend: Fallback on RR algo if balance on source is impossible

If the loadbalancing is performed on the source IP address, an internal
error was returned on error. So for an applet on the client side (for
instance an SPOE applet) or for a client connected to a unix socket, an
internal error is returned.

However, when other LB algos fail, a fallback on round-robin is
performed. There is no reson to not do the same here.

This patch should fix the issue #1797. It must be backported to all
supported versions.