remove SIBackendInit return value

SIBackendInit returns a flag indicating whether it worked or not.  Since
there is only one caller and it dies with a FATAL error when
SIBackendInit failed, it seems better to move the elog and remove the
return value, per this patch.

--
Alvaro Herrera                  http://www.amazon.com/gp/registry/5ZYLFMCVHXC
"Always assume the user will do much worse than the stupidest thing
you can imagine."                                (Julien PUYDT)