Option to fail instead of falling back to zarr-python

[tomwhite]

I'm currently testing zarrs-python with Cubed (cubed-dev/cubed#789) and it would be useful if there was a way to fail immediately rather than silently falling back to zarr-python. Some kind of strict mode config option perhaps? This could be enabled when running tests and would help adopters see where there are gaps in support.

[tomwhite]

Quick update: zarrs-python passed all tests first time without any failures, and was 16% faster than regular zarr-python on one Cubed benchmark running locally.

Nice work on zarrs!