Avoid redundant __contains__

[dcherian]

Let's try grabbing the array.json and group.json files, and check for *NotFoundError, instead of using contains first.

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)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.98%. Comparing base (d0fb875) to head (58209e8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1739      +/-   ##
==========================================
- Coverage   99.99%   99.98%   -0.01%     
==========================================
  Files          38       38              
  Lines       14610    14614       +4     
==========================================
+ Hits        14609    14612       +3     
- Misses          1        2       +1     

Files	Coverage Δ
zarr/hierarchy.py	99.79% <100.00%> (-0.21%)	⬇️

[jhamman]

@dcherian - this is so so great. Zarr-python is 9 kinds of defensive - often times when it doesn't need to be. This is a prime example. Whoop!