Drop an informative error message for towncrier failures

.github/workflows/towncrier-run.yml

@@ -67,4 +67,21 @@ jobs:
       run: python -m pip install ${{ inputs.tox-source }}
 
     - name: Run towncrier check
+      id: tox
       run: tox -e towncrier-check
+
+    - name: Towncrier Check Resolution Instructions
+      if: always() && steps.tox.outcome == 'failure'
+      run: |
+        echo '::error::*************************************************************************'
+        echo '::error::** Towncrier check failed for missing release note                     **'
+        echo '::error::*************************************************************************'
+        echo '::error:: '
+        echo '::error::A release note for the PR is missing from the `changes` directory.'
+        echo '::error:: '
+        echo '::error::Visit the documentation below to add one:'
+        echo '::error:: '
+        echo '::error::    https://toga.readthedocs.io/en/latest/how-to/contribute-code.html#add-change-information-for-release-notes'
+        echo '::error:: '
+        echo '::error::*************************************************************************'
+        exit 1