Properly exit on error in block to block test

tests/message_exchange_block_per_block.nu

 use ../swarm.nu *
 use ../app.nu
 use std assert
+use help_func/exit_func.nu exit_on_error
 
 # define variables
 let SWARM = swarm create 2
@@ -108,7 +109,7 @@ try {
     swarm kill --no-shell
 
     print "Checking the difference between the original and reconstructed file"
-    let difference = diff $"($output_dir)/../($res_filename)" $test_file
+    let difference = {diff $"($output_dir)/../($res_filename)" $test_file} | exit_on_error | get stdout
     if $difference == "" {
         print "Test successful !"
     } else {