qt: lnurlw: catch UserCancelled on lnurlw withdrawal

Catch the UserCancelled exception if the user cancels the lnurlw
coroutine dialog during the withdrawal.

electrum/gui/qt/send_tab.py

@@ -1002,3 +1002,5 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
             self.window.run_coroutine_dialog(coro, _("Requesting lightning withdrawal..."))
         except LNURLError as e:
             self.show_error(f"{_('Failed to request withdrawal')}:\n{str(e)}")
+        except UserCancelled:
+            pass