Fix NameError in sweep error handler lambda

Python 3.13 deletes exception variables after the except block.
Capture the error string before passing to the lambda.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

testbench/gui.py

@@ -1159,8 +1159,9 @@ class TestbenchGUI(tk.Tk):
                     self._console(msg, "warn")
                 self.after(0, lambda: self._svi_status.config(text=msg))
             except Exception as e:
+                err_msg = f"Error: {e}"
                 self._console(f"Sweep error: {e}", "error")
-                self.after(0, lambda: self._svi_status.config(text=f"Error: {e}"))
+                self.after(0, lambda: self._svi_status.config(text=err_msg))
             finally:
                 self.after(0, self._sweep_vi_done)