Update GlobalAlgBlkUnpacker to throw error in the case of headers rec…

…eived in the wrong order

EventFilter/L1TRawToDigi/plugins/implementations_stage2/GlobalAlgBlkUnpacker.cc

@@ -46,6 +46,13 @@ namespace l1t {
           GlobalAlgBlk talg = GlobalAlgBlk();
           res_->push_back(bx, talg);
         }
+        //If this is not the first block, but the vector is empty, something has gone wrong (corrupted data)
+        else if (res_->isEmpty(bx))
+          throw cms::Exception("InvalidGlobalAlgBlkBxCollection")
+              << "The GlobalAlgBlk unpacker result vector is empty, but is not receiving the first expected header "
+                 "ID! This may be due to corrupted, or poorly formatted events.\n"
+              << "uGTBoard: " << uGTBoard << "\nBX: " << bx << "\nFirst expected block: " << initialBlkID
+              << "\nReceived block: " << block.header().getID();
 
         //fetch
         GlobalAlgBlk alg = res_->at(bx, 0);