bug#34477: Fleeting docview warning revealed on slow computers

[積丹尼 Dan Jacobson]

On a super slow computer, doc-view.el's

    ;; Typically happens if the conversion process somehow
    ;; failed.  Better not signal an error here because it
    ;; could prevent a subsequent reconversion from fixing
    ;; the problem.
    (concat "Cannot display this page!\n"
            "Maybe because of a conversion failure!"))))

ends up appearing just before the document is displayed.

Testing on fast computers you will never see the problem.

Better replace the message with "One moment please..."

Thanks.