Fail the documentation build if there are any Doxygen warnings

Check for Doxygen warnings, that are, for the most part, actually
errors, to ensure that they don't creep back in after they were recently
fixed.

.github/workflows/docs_update.yml

@@ -30,6 +30,13 @@ jobs:
         working-directory: docs/doxygen
         run: |
           ./regen.sh php
+          if [[ -s doxygen.log ]]; then
+            echo '*** There were warnings during docs generation: ***'
+            echo '-----------------------------------------------------------'
+            cat doxygen.log
+            echo '-----------------------------------------------------------'
+            exit 1
+          fi
 
       - name: Upload
         if: github.ref == 'refs/heads/master'