fix open_array for mode r+

[d-v-b]

Raises FileNotFoundError if open_array is invoked with mode=r+ when no extant array is found.

Fixes #2491

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)