Raise helpful errors when opening a group with wrong node type.

[dstansby]

Fixes #3114

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.54%. Comparing base (df161b2) to head (5853a15).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zarr/core/group.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3444   +/-   ##
=======================================
  Coverage   60.53%   60.54%           
=======================================
  Files          81       81           
  Lines        9694     9701    +7     
=======================================
+ Hits         5868     5873    +5     
- Misses       3826     3828    +2     

Files with missing lines	Coverage Δ
src/zarr/core/group.py	70.51% <71.42%> (+<0.01%)	⬆️

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