Make 'F' error codes from flake8 be errors (microsoft#976)

Some are runtime errors (e.g. `break` outside of a loop), while others are just warnings (e.g. unused imports). Reverts part of 7894ae6

Author: brettcannon

package.json

@@ -1353,7 +1353,7 @@
  },
  "python.linting.flake8CategorySeverity.F": {
  "type": "string",
- "default": "Warning",
+ "default": "Error",
  "description": "Severity of Flake8 message type 'F'.",
  "enum": [
  "Hint",