enable flake8 basic check

.github/workflows/test-dumogenerator.yml

@@ -31,8 +31,8 @@ jobs:
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: Lint with flake8
       run: |
-        # still continue if there are Python syntax errors or undefined names
-        flake8 . --count --exit-zero --select=E9,F63,F7,F82 --show-source --statistics
+        # exit if there are Python syntax errors or undefined names
+        flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
         # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
         flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
     - name: run dumpgenerator