More progress on integrating ariadne's error reporting

src/builtin/shift.rs

@@ -22,11 +22,7 @@ pub fn shift(node: Node, job: &mut JobBldr) -> ShResult<()> {
 
   if let Some((arg, span)) = argv.next() {
     let Ok(count) = arg.parse::<usize>() else {
-      return Err(ShErr::full(
-        ShErrKind::ExecFail,
-        "Expected a number in shift args",
-        span,
-      ));
+      return Err(ShErr::at(ShErrKind::ExecFail, span, "Expected a number in shift args"));
     };
     for _ in 0..count {
       write_vars(|v| v.cur_scope_mut().fpop_arg());