Small fix in prepare_target: catch PathNotFoundError.

[alxmrs]

Prepare target checks if a Zarr exists by attempting to open a Zarr and then creating one when an error occurs. I've found that in addition to the zarr.errors.GroupNotFoundError, the zarr.errors.PathNotFoundError needs to be caught in order to not break the xarray_zarr.py pipeline.

[rabernat]

Looks like a prefect release has broken all of our test suite.

ImportError while loading conftest '/home/runner/work/pangeo-forge-recipes/pangeo-forge-recipes/tests/conftest.py'.
tests/conftest.py:29: in <module>
    from prefect.executors import DaskExecutor
E   ModuleNotFoundError: No module named 'prefect.executors'

I think the solution is probably to pin an older version of prefect in our CI. Since we are planning on removing prefect support anyway, I don't see any reason not to do this as a short term fix to get the tests passing.

[rabernat]

Prefect test issue is being fixed in #398. In the meantime, LGTM.