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

…eived in the wrong order

EventFilter/L1TRawToDigi/plugins/implementations_stage2/GlobalAlgBlkUnpacker.cc

@@ -40,6 +40,12 @@ namespace l1t {
       // Loop over multiple BX and then number of EG cands filling collection
       int numBX = 0;  //positive int to count BX
       for (int bx = firstBX; bx <= lastBX; bx++) {
+        // Check to make sure that we are receiving the first expected header here
+        // If we are not, throw a more informative error than the failure of the following "at->" call
+        if (res_->size(bx) == 0 and block.header().getID() != initialBlkID)
+          throw(
+              std::runtime_error("The GlobalAlgBlk unpacker result vector is size 0, but is not receiving the first "
+                                 "expected header ID! This may be due to corrupted, or poorly formatted events."));
         // If this is the first block on first board, instantiate GlobalAlg so it is there to fill from mult. blocks
         if (block.header().getID() == initialBlkID && uGTBoard == 0) {
           LogDebug("L1T") << "Creating GT Algorithm Block for BX =" << bx << std::endl;