ci/validate-pr: Use `::error::` command to print errors

This will make Github render the log line as an error.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

.github/workflows/validate-pr.yml

@@ -11,7 +11,7 @@ jobs:
       - name: Missing `area/` label
         if: contains(join(github.event.pull_request.labels.*.name, ','), 'impact/') && !contains(join(github.event.pull_request.labels.*.name, ','), 'area/')
         run: |
-          echo "Every PR with an \`impact/*\` label should also have an \`area/*\` label"
+          echo "::error::Every PR with an 'impact/*' label should also have an 'area/*' label"
           exit 1
       - name: OK
         run: exit 0
@@ -32,13 +32,13 @@ jobs:
           desc=$(echo "$block" |  awk NF)
 
           if [ -z "$desc" ]; then
-            echo "Changelog section is empty. Please provide a description for the changelog."
+            echo "::error::Changelog section is empty. Please provide a description for the changelog."
             exit 1
           fi
 
           len=$(echo -n "$desc" | wc -c)
           if [[ $len -le 6 ]]; then
-            echo "Description looks too short: $desc"
+            echo "::error::Description looks too short: $desc"
             exit 1
           fi