> I think it is cleaner to bind
 > buffer-read-only and buffer-undo-list only once.

On the one hand I don't want to compromise the original behavior for
`revert-buffer-insert-file-contents-function' non-nil.  On the other I
don't want to bind `buffer-undo-list' when generating the "File no
longer exists ..." errors.  With `debug-on-error' non-nil, switching to
*Backtrace* would leave me without undo information again.  Hence the -
admittedly ugly - two bindings.  I added a comment to explain this.