bug#64516: [PATCH] docview: Only enable imenu when supported

[Eli Zaretskii <eliz@gnu.org>]

> From: Morgan Smith <Morgan.J.Smith@outlook.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  64516@debbugs.gnu.org,  jao@gnu.org
> Date: Tue, 11 Jul 2023 14:22:49 -0400
> 
> I apologize for my "oops I figured it out immediately after I sent the
> email" syndrome.  I was trying to find the answer in the bug reports
> when the answer was in the code and quite obvious.
> 
> Anyways here is a patch that unconditionally enables imenu but checks
> the return value of mutool and will raise an imenu-unavailable-error
> when it is not successful.  However, that error stops docview from
> displaying the document so I decided to put the setup function in a hook
> so the error occurs after the document is displayed.
> 
> I looked briefly for documentation on how errors work in hooks but could
> not find any.  I don't want this error to prevent the other hooks from
> being run (which would be user defined hooks.  we don't currently use
> this hook).

Tassilo, is this okay, in your opinion?