terminate if no files found

sort.cpp

@@ -32,6 +32,11 @@ int main( int argc, char* argv[] ){
     vector<string> input;
     findFiles(run, input);
 
+    if (input.size() == 0) {
+        cout << "Unable to find any files with run number " << run << endl;
+        return 0;
+    }
+
     vector<shared_ptr<VKRawReader>> readers;
     for (auto& i : input) {
         auto r = make_shared<VKRawReader> (i);