templater: raise error for unknown func

Previously, if a template '{foo()}' was given, the buildfunc would not be able
to match it and hit a code path that would not return so it would error out
later in the templater stating that NoneType was not iterable. This patch makes
sure that a proper error is raised so that the user can be informed.

Tests have been updated.

mercurial/templater.py

@@ -212,6 +212,7 b' def buildfunc(exp, context):'
             raise error.ParseError(_("filter %s expects one argument") % n)
         f = context._filters[n]
         return (runfilter, (args[0][0], args[0][1], f))
+    raise error.ParseError(_("unknown function '%s'") % n)
 
 def date(context, mapping, args):
     if not (1 <= len(args) <= 2):

tests/test-command-template.t

@@ -1416,6 +1416,12 b' Behind the scenes, this will throw Value'
   abort: template filter 'datefilter' is not compatible with keyword 'author'
   [255]
 
+Thrown an error if a template function doesn't exist
+
+  $ hg tip --template '{foo()}\n'
+  hg: parse error: unknown function 'foo'
+  [255]
+
   $ cd ..