repmgrd: check shared library is loaded

If this isn't the case, "repmgrd" will appear to run but not handle
failover correctly.

Address GitHub #337.

repmgr.c

@@ -124,7 +124,6 @@ _PG_init(void)
 {
 	elog(DEBUG1, "repmgr init");
 
-	/* error here? */
 	if (!process_shared_preload_libraries_in_progress)
 		return;