show an error in tests/hello_world_file.cpp if we can't open the file; closes #2389

Author: kripken

tests/hello_world_file.cpp

@@ -1,6 +1,10 @@
 #include <stdio.h>
 int main() {
   FILE *file = fopen("tests/hello_world_file.txt", "rb");
+  if (!file) {
+    printf("cannot open file\n");
+    return 1;
+  }
   while (!feof(file)) {
     char c = fgetc(file);
     if (c != EOF) {