update error message to actually be helpful

src/run.rs

@@ -91,7 +91,7 @@ pub fn run_callback(ast: &Seg, syms: &mut SymTable) ->  Result<Ctr, Traceback> {
             }
         } else {
             Err(start_trace(
-                ("<call script>", "binary called, unimplemented!")
+                ("<call script>", "expected a relish script with a .rls extension")
                     .into()))
         }
     } else {