keg: tell people what command will remove keg.

This will work regardless of permissions.

Library/Homebrew/keg.rb

@@ -311,7 +311,10 @@ class Keg
     remove_old_aliases
     remove_oldname_opt_record
   rescue Errno::ENOTEMPTY
-    ofail "Could not remove #{path}! Check its permissions."
+    odie <<~EOS
+      Could not remove #{name} keg! Do so manually:
+        sudo rm -rf #{path}
+    EOS
   end
 
   def unlink(mode = OpenStruct.new)