bug#17518: 24.4.50; Warning should mention file, if possible: "`lexical-binding' at end of file unreliable"

[Drew Adams]

I performed a tags search using an old TAGS file.  *Lots* of files were
searched, and up popped a *Warnings* buffer with this single message:

  Warning (:warning): : `lexical-binding' at end of file unreliable

If the file is known when the warning is issued then let's please
include its name in the warning.

And what is with this part?

  Warning (:warning): :

What kind of incantation is that?  Can we be more helpful to users here?
Are they supposed to guess why "(:warning)" appears here, or why there
are so many colons?


In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-04-29 on ODIEONE
Bzr revision: 117031 monnier@iro.umontreal.ca-20140429151607-qnkgbymwfaj5ut08
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''