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):
      ...

mercurial/templater.py

@@ -547,12 +547,8 b' def label(context, mapping, args):'
         # i18n: "label" is a keyword
         raise error.ParseError(_("label expects two arguments"))
 
+    ui = mapping['ui']
     thing = evalstring(context, mapping, args[1])
-
-    ui = mapping.get('ui', '')
-    if isinstance(ui, str):
-        return thing
-
     # preserve unknown symbol as literal so effects like 'red', 'bold',
     # etc. don't need to be quoted
     label = evalstringliteral(context, mapping, args[0])