Make Vale warnings fail the pipeline

Signed-off-by: Karel van Klink <karel.vanklink@geant.org>

docs/.gitlab-ci.yml

@@ -39,4 +39,6 @@ lint-documentation:
     - vale sync
 
   script:
-    - vale $CI_PROJECT_DIR/docs/build/html/_sources
+    - RESULT=$(vale $CI_PROJECT_DIR/docs/build/html/_sources)
+    - echo "$RESULT"
+    - if echo "$RESULT" | tail -n 1 | grep -q "✖" ; then echo "Please fix all Vale errors and warnings" && exit 1 ; else exit 0 ; fi