Fix updating quantizers between pq, bq, and sq with hnsw index

[tsmith023]

• Forbid updating the quantizer of an index if it already has one configured (should be on the server but adding to client for now)
• Add unit tests for quantizer merge_with_existing method
• Any errors with updating quantizers for unsupported indices, e.g. pq and sq with flat, are returned from the server as UnexpectedStatusCodeError exceptions

Closes #1170

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.30%. Comparing base (f19987f) to head (772c25e).
Report is 3 commits behind head on dev/1.26.

Additional details and impacted files

@@             Coverage Diff              @@
##           dev/1.26    #1171      +/-   ##
============================================
- Coverage     94.31%   94.30%   -0.02%     
============================================
  Files           213      215       +2     
  Lines         20490    20553      +63     
============================================
+ Hits          19325    19382      +57     
- Misses         1165     1171       +6     

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