Check return code from 'pipenv install' command.

Otherwise this can fail partially and cause problems later in the build:
2024-03-14T00:29:16.4350190Z [pipenv.exceptions.InstallError]: ERROR: Could not build wheels for typogrify, which is required to install pyproject.toml-based projects
...
2024-03-14T01:01:25.0548260Z gi-docgen generate -C /Users/runner/gtk/source/librsvg-2.57.0/doc/librsvg.toml --content-dir=/Users/runner/gtk/source/librsvg-2.57.0/doc ../Rsvg-2.0.gir
...
2024-03-14T01:01:25.4160950Z   File "/Users/runner/.new_local/share/virtualenvs/etc-jUmrpTpB/lib/python3.11/site-packages/gidocgen/utils.py", line 14, in <module>
2024-03-14T01:01:25.4164410Z     from typogrify.filters import typogrify
2024-03-14T01:01:25.4165840Z ModuleNotFoundError: No module named 'typogrify'

Context: Xpra-org#41 (comment)

gtk-osx-setup.sh

@@ -249,7 +249,7 @@ if test -d "$SDKROOT"; then
     export CFLAGS="-isysroot $SDKROOT -I$SDKROOT/usr/include"
 fi
 
-$PIPENV install
+$PIPENV install || exit 1
 
 BASEURL="https://raw.githubusercontent.com/Xpra-org/gtk-osx-build/master"