--- a/hgext3rd/evolve/rewriteutil.py	Sun Jul 23 19:53:51 2017 +0200
+++ b/hgext3rd/evolve/rewriteutil.py	Sun Jul 23 19:45:28 2017 +0200
@@ -68,7 +68,8 @@
         raise error.Abort(msg, hint=hint)
     newunstable = disallowednewunstable(repo, revs)
     if newunstable:
-        msg = _("cannot fold chain not ending with a head or with branching")
+        msg = _("%s will orphan %i descendants")
+        msg %= (action, len(newunstable))
         hint = _("see 'hg help evolution.instability'")
         raise error.Abort(msg, hint=hint)
 

--- a/tests/test-fold.t	Sun Jul 23 19:53:51 2017 +0200
+++ b/tests/test-fold.t	Sun Jul 23 19:45:28 2017 +0200
@@ -163,11 +163,11 @@
   > evolution = createmarkers, allnewcommands
   > EOF
   $ hg fold --from 'desc("r4")'
-  abort: cannot fold chain not ending with a head or with branching
+  abort: fold will orphan 1 descendants
   (see 'hg help evolution.instability')
   [255]
   $ hg fold --from 'desc("r3")::desc("r11")'
-  abort: cannot fold chain not ending with a head or with branching
+  abort: fold will orphan 1 descendants
   (see 'hg help evolution.instability')
   [255]
 

--- a/tests/test-metaedit.t	Sun Jul 23 19:53:51 2017 +0200
+++ b/tests/test-metaedit.t	Sun Jul 23 19:45:28 2017 +0200
@@ -118,7 +118,7 @@
   (587528abfffe will become unstable and new unstable changes are not allowed)
   [255]
   $ hg metaedit 'desc(A)::desc(B)' --fold --config 'experimental.evolution=createmarkers, allnewcommands'
-  abort: cannot fold chain not ending with a head or with branching
+  abort: fold will orphan 4 descendants
   (see 'hg help evolution.instability')
   [255]
   $ hg metaedit --user foobar