ci: accurate error reporting on lint check (#1089)

.github/workflows/go.yml

@@ -29,7 +29,7 @@ jobs:
       run: |
         go vet -stdmethods=false $(go list ./...)
         go install mvdan.cc/gofumpt@latest
-        test -z "$(gofumpt -s -l -extra .)"
+        test -z "$(gofumpt -s -l -extra .)" || echo "Please run 'gofumpt -l -w -extra .'"
 
     - name: Test
       run: go test -race -coverprofile=coverage.txt -covermode=atomic ./...