Issue #5751 - gui/siegemanager - cleaner error logging if no siege engine selected

[Halavus]

If this PR makes an externally-visible change in behavior, please add an appropriate line to changelog.txt.

[Halavus]

pre-commit.ci autofix

[chdoc]

I think this is fundamentally the wrong way to fix the problem. All it does is change the error instead of avoiding it. The underlying issue is that the labels for setting the siege engines stance are enabled even if there is no siege engine selected. Thus, the right way to address this is to provide a function/callback that checks whether the labels should be enabled, as has been done here for example:

scripts/idle-crafting.lua

Lines 576 to 580 in 56c934e

	enabled = function()
	local bld = dfhack.gui.getSelectedBuilding(true)
	if not bld then return end
	return not invalidProfile(bld)
	end,