Warn about invalid index entries

[Glenn Morris]

Hi,

The Texinfo manual says:

     Caution: Do not use a colon in an index entry. In Info, a colon
     separates the menu entry name from the node name, so a colon in
     the entry itself confuses Info.

Yet makeinfo happily accepts such index entries with no complaint.
(For example, I have just discovered that about 1% of index entries in
the Emacs manuals are useless because of this reason.)

Didn't this use to cause a warning at some point? Eg
  http://lists.gnu.org/archive/html/bug-texinfo/2002-11/msg00001.html

Oh, from the 4.13 source I see the warning got ifdef'd out at some point. :(

Yours-expecting-to-get-
  http://lists.gnu.org/archive/html/bug-texinfo/2013-08/msg00009.html
-all-over-againly,