diff -r 58b36e9ea783 -r 973959fbe8ec mercurial/scmutil.py
--- a/mercurial/scmutil.py	Tue May 31 19:16:18 2011 +0200
+++ b/mercurial/scmutil.py	Tue May 31 19:16:25 2011 +0200
@@ -698,6 +698,6 @@
     requirements = set(opener.read("requires").splitlines())
     for r in requirements:
         if r not in supported:
-            raise error.RequirementError(
-                _("requirement '%s' not supported") % r)
+            raise error.RequirementError(_("unknown repository format: "
+                "requires feature '%s' (upgrade Mercurial)") % r)
     return requirements