General refactoring

[lintool]

• Refactored SearchHnswDenseVectors and SearchInvertedDenseVectors, cleaning up code paths and main error handling.
• Added test cases, cleaned up existing test cases.
• Other minor cleanup

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (95a546c) 62.92% compared to head (0519a94) 63.40%.

Files	Patch %	Lines
...io/anserini/search/SearchInvertedDenseVectors.java	95.12%	0 Missing and 2 partials ⚠️
...ava/io/anserini/search/SearchHnswDenseVectors.java	97.67%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2269      +/-   ##
============================================
+ Coverage     62.92%   63.40%   +0.47%     
- Complexity     1271     1286      +15     
============================================
  Files           196      196              
  Lines         11423    11463      +40     
  Branches       1453     1457       +4     
============================================
+ Hits           7188     7268      +80     
+ Misses         3712     3672      -40     
  Partials        523      523              

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[ArthurChen189]

lgtm!