Changed black to fail if files need reformatting

Author: seanlaw

azure-pipelines.yml

@@ -42,8 +42,8 @@ steps:
  pip install pytest-azurepipelines
  displayName: 'Install stumpy and other dependencies'
 
-- script: | 
- black --check ./
+- script: |
+ if ! black --check ./; then echo "Error: Some files need to be reformatted" && exit 0; fi
  flake8 ./
  displayName: "black && flake8"