templater: make label() just fail if ui object isn't available Silent failure hides bugs and makes it harder to track down the issue. It's worse than raising exception. In future patches, I plan to sort out template functions that require 'ui', 'ctx', 'fctx', etc. so that incompatible functions are excluded and the doc can say in which context these functions are usable. @templatefunc('label', requires=('ui',)) def label(context, mapping, args): ...