Improve non-existent executable test

Make it a bit more obvious whether the non-existent executable test has
passed. Still to do: make an actual test suite that will exit 1 if it
doesn't succeed

Author: hdgarrood

test/Main.purs

@@ -19,7 +19,7 @@ main = do
 
   log "emits an error if executable does not exist"
   nonExistentExecutable $ do
-    log "all good."
+    log "nonexistent executable: all good."
 
   log "doesn't perform effects too early"
   ls <- spawn "ls" ["-la"] defaultSpawnOptions