gitlab runner: print reason for skipping doc builds to stderr

It does not matter though... no idea how to set the reason for failing
runs.

.gitlab-ci.yml

@@ -11,7 +11,7 @@ build-latex:
   - latex
   # FIXME: do it sanely when this is resolved: https://gitlab.com/gitlab-org/gitlab-ce/issues/19232
   script:
-  - '{ git diff --quiet --exit-code HEAD^ -- docs/technical_reference_manual/ docs/tex_common/ docs/user_guide && printf "No relevant changes in docs/.\n" && exit 1; } || make -C docs/ ug trm'
+  - '{ git diff --quiet --exit-code HEAD^ -- docs/technical_reference_manual/ docs/tex_common/ docs/user_guide && printf "No relevant changes in docs/.\n" >&2 && exit 1; } || make -C docs/ ug trm'
   allow_failure: true
   artifacts:
     paths: