stream: improve error handling of ssn pool

With large number of threads the default memcap leads to pool setup
failures. Make sure these are reported properly so that the user
knows what is going on.

Bug: #2242

src/stream-tcp.c

@@ -4973,8 +4973,10 @@ TmEcode StreamTcpThreadInit(ThreadVars *tv, void *initdata, void **data)
         SCLogDebug("pool size %d, thread ssn_pool_id %d", PoolThreadSize(ssn_pool), stt->ssn_pool_id);
     }
     SCMutexUnlock(&ssn_pool_mutex);
-    if (stt->ssn_pool_id < 0 || ssn_pool == NULL)
+    if (stt->ssn_pool_id < 0 || ssn_pool == NULL) {
+        SCLogError(SC_ERR_MEM_ALLOC, "failed to setup/expand stream session pool. Expand stream.memcap?");
         SCReturnInt(TM_ECODE_FAILED);
+    }
 
     SCReturnInt(TM_ECODE_OK);
 }