`next-file' in etags.el signaling error to denote end of operation

["Tobias C. Rittweiler" <tcr@freebits.de>]

The function `next-file' in etags.el signals an unnamed error when the
end of `next-file-list' is reached.

This makes it not all that easy to call `tags-query-replace' multiple
times in a row programmatically as far as I can see.

An easy solution would be to make `next-file' signal a named condition
that can then be caught programmatically.

I'll post this to the bug tracker unless someone can point out that I'm
missing something.

  -T.