Show zensical errors in workflow status

Unfortunately the build workflow uses an ages old zensical version
which does not return an error status if rendering fails.
Instead now generate an error status if a line starting with
"Error:" is found in the zensical message output.

.gitea/workflows/deploy-pages.yml

@@ -18,7 +18,11 @@ jobs:
       - name: Build page
         run: |
             sed -i 's|repo_url[[:space:]]*=[[:space:]]*".*"|repo_url = "${{ github.server_url }}/${{ github.repository }}"|' zensical.toml
-            /opt/python-env/bin/zensical build
+            echo "Zensical version: `/opt/python-env/bin/zensical --version`"
+            # As long as we use ages old Zensical version 0.0.11,
+            # error status return does not work. Let's do our own.
+            #/opt/python-env/bin/zensical build
+            ! /opt/python-env/bin/zensical build 2>&1 | tee /dev/stderr | grep -q '^Error:'
       
       - name: Configure Git
         run: |