templater: show slightly better hint on map operation error

It was super hard to find where the templater went wrong if a keyword was
evaluated to '' or None.

mercurial/templateutil.py

@@ -753,14 +753,22 b' def _iteroverlaymaps(context, origmappin'
         lm['index'] = i
         yield lm
 
-def _applymap(context, mapping, d, targ):
-    for lm in _iteroverlaymaps(context, mapping, d.itermaps(context)):
+def _applymap(context, mapping, d, darg, targ):
+    try:
+        diter = d.itermaps(context)
+    except error.ParseError as err:
+        sym = findsymbolicname(darg)
+        if not sym:
+            raise
+        hint = _("keyword '%s' does not support map operation") % sym
+        raise error.ParseError(bytes(err), hint=hint)
+    for lm in _iteroverlaymaps(context, mapping, diter):
         yield evalrawexp(context, lm, targ)
 
 def runmap(context, mapping, data):
     darg, targ = data
     d = evalwrapped(context, mapping, darg)
-    return mappedgenerator(_applymap, args=(mapping, d, targ))
+    return mappedgenerator(_applymap, args=(mapping, d, darg, targ))
 
 def runmember(context, mapping, data):
     darg, memb = data

tests/test-command-template.t

@@ -3213,6 +3213,7 b' Test new-style inline templating:'
 
   $ hg log -R latesttag -r tip -T '{rev % "a"}\n'
   hg: parse error: 11 is not iterable of mappings
+  (keyword 'rev' does not support map operation)
   [255]
   $ hg log -R latesttag -r tip -T '{get(extras, "unknown") % "a"}\n'
   hg: parse error: None is not iterable of mappings