Fix pyfetch when there is a cors error. As pointed out by @ryanking13 in https://github.com/pyodide/pyodide/pull/2589#discussion_r878950933:
If `pyfetch` generates a cors error, it is thrown as a `JsException` rather than as an `OSError`.

### Checklists

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests

package:
  name: cffi
  version: 1.15.0
requirements:
  run:
    - pycparser
source:
  url: https://files.pythonhosted.org/packages/00/9e/92de7e1217ccc3d5f352ba21e52398372525765b2e0c4530e6eb2ba9282a/cffi-1.15.0.tar.gz
  sha256: 920f0d66a896c2d99f0adbb391f990a84091179542c205fa53ce5787aff87954
  patches:
    - patches/libffi-config.patch
build:
  cross-build-env: true
test:
  imports:
    - cffi
about:
  home: http://cffi.readthedocs.org
  PyPI: https://pypi.org/project/cffi
  summary: Foreign Function Interface for Python calling C code.
  license: MIT

package:
  name: cffi
  version: 1.15.0
requirements:
  run:
    - pycparser
source:
  url: https://files.pythonhosted.org/packages/00/9e/92de7e1217ccc3d5f352ba21e52398372525765b2e0c4530e6eb2ba9282a/cffi-1.15.0.tar.gz
  sha256: 920f0d66a896c2d99f0adbb391f990a84091179542c205fa53ce5787aff87954
  patches:
    - patches/libffi-config.patch
build:
  cross-build-env: true
test:
  imports:
    - cffi
about:
  home: http://cffi.readthedocs.org
  PyPI: https://pypi.org/project/cffi
  summary: Foreign Function Interface for Python calling C code.
  license: MIT
