[libadwaita/wip/exalm/dist-script] dist-data: Error out if it can't find docs

[Alexander Mikhaylenko <alexm src gnome org>]

commit f7dcb13ccb0d77d06174e5daf10d3d9212987672
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Fri Aug 5 19:01:07 2022 +0400

    dist-data: Error out if it can't find docs
    
    The purpose of that script is to include pre-built docs and stylesheets
    into the tarball. However, while building styles is automated and the
    script does it on its own, the docs need to be manually built beforehand.
    This step is easy to miss, so visibly fail instead of continuing with an
    incomplete tarball.

 build-aux/meson/dist-data.py | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/build-aux/meson/dist-data.py b/build-aux/meson/dist-data.py
index 86f38c2b..2df333a9 100644
--- a/build-aux/meson/dist-data.py
+++ b/build-aux/meson/dist-data.py
@@ -3,6 +3,7 @@
 import os
 import shutil
 import subprocess
+import sys
 
 from pathlib import PurePath
 
@@ -32,3 +33,5 @@ for reference in references:
     if os.path.isdir(src_path):
         dst_path = os.path.join(distroot, reference)
         shutil.copytree(src_path, dst_path)
+    else:
+        sys.exit("Documentation not found. Please pass -Dgtk_doc=true and to Meson and run ninja once.")