relax the enforce_exists check because it will be done later

[toloudis]

Link to Relevant Issue

This pull request resolves #41

Description of Changes

Related to #40 .
companion PR to bioio-ome-zarr: bioio-devs/bioio-ome-zarr#20
Don't enforce_exists until specific Readers are trying to check for file support.

A side effect of this is that some bad file paths will raise a different kind of exception. (see modified unit test)

BONUS FIX: macos-latest CI build was failing due to M1 support for imageio-ffmpeg.

[evamaxfield]

In general I approve. Still wish we could give back a specific "this file wasn't found" vs "this file isn't supported" error but I understand

[toloudis]

In general I approve. Still wish we could give back a specific "this file wasn't found" vs "this file isn't supported" error but I understand

There is a way to do this - it's possible for the ReaderClass.is_supported_image to raise it now, so I added a couple lines to catch and rethrow it.
No unit test gets this because there are no plugin readers installed when we run the unit tests.
But I verified by adding a tiff plugin and sending a fake .tiff file path and it did raise FileNotFound