throw upon empty file load pattern

pub/lib/file_in.cpp

@@ -41,6 +41,8 @@ void NFileIn::load(void)
 {
     string pattern = sask("Load file(s)");
     vector<string> fNames = triv::glob_file_list(pattern, "yda y08 a01 i96");
+    if (!fNames.size())
+        throw "Found no files that match the pattern '" + pattern + "'";
     for (string fnam : fNames) {
         try {
             string fdir, fshort, fext;