revset: factor out composing error message for ParseError to reuse

This patch defines the composing function not in "ParseError" class but
in "revset" module, because:

  - "_()" shouldn't be used in "ParseError", to avoid adding "from
    i18n import _" i18n" to "error" module

  - generalizing message composition of"ParseError" for all code paths
    other than revset isn't the purpose of this patch

    we should also take care of showing "unexpected leading
    whitespace" for some code paths, to generalize widely.

mercurial/revset.py

@@ -239,6 +239,14 b' def tokenize(program, lookup=None, symin'
         pos += 1
     yield ('end', None, pos)
 
+def parseerrordetail(inst):
+    """Compose error message from specified ParseError object
+    """
+    if len(inst.args) > 1:
+        return _('at %s: %s') % (inst.args[1], inst.args[0])
+    else:
+        return inst.args[0]
+
 # helpers
 
 def getstring(x, err):
@@ -2146,10 +2154,7 b' class revsetalias(object):'
             # Check for placeholder injection
             _checkaliasarg(self.replacement, self.args)
         except error.ParseError, inst:
-            if len(inst.args) > 1:
-                self.error = _('at %s: %s') % (inst.args[1], inst.args[0])
-            else:
-                self.error = inst.args[0]
+            self.error = parseerrordetail(inst)
 
 def _getalias(aliases, tree):
     """If tree looks like an unexpanded alias, return it. Return None