bug#26650: 26.0.50; Protect *Backtrace* from accidental killing

[Tino Calancha]

I often kills the *Backtrace* buffer; then, errors won't show
*Backtrace* again in this session.
Could *Backtrace* be guarded with something like `edebug-kill-buffer'
in kill-buffer-hook?

emacs -Q
;; eval following form:
(error "hi") ; This jump to *Backtrace*
q ; Back to top level

;; Eval it again:
(error "hi") ; This time kill *Backtrace*
C-x k RET

;; Eval it once more:
(error "hi")
;; 'hi' is printed in echo area, but *Backtrace* is not shown.

In GNU Emacs 26.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2017-04-25 built
Repository revision: 5b0fdefb4ca54b0d3dac3047ac1e4b380beb6ba7