Raise an ImportError for HDFVirtualBackend without h5py installed.

[sharkinsspatial]

• Closes Confusing error message when following usage guide without h5py installed #470
• Tests added
• Tests passing

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.66%. Comparing base (fd3a916) to head (0e89f2d).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
virtualizarr/readers/hdf/hdf.py	50.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (50.00%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #481      +/-   ##
===========================================
- Coverage    88.70%   88.66%   -0.05%     
===========================================
  Files           28       28              
  Lines         1753     1755       +2     
===========================================
+ Hits          1555     1556       +1     
- Misses         198      199       +1     

Files with missing lines	Coverage Δ
virtualizarr/readers/hdf/hdf.py	94.84% <50.00%> (-0.47%)	⬇️

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