add IT cases for filtering out non-server exceptions for HC detector

[ylwu-amzn]

Issue #, if available:

Description of changes:
1.Add more IT cases for filtering out non-server exceptions for HC detector.
2.Address comments #344 (comment)

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[codecov]

Codecov Report

Merging #348 (ae083ca) into master (7df38a1) will increase coverage by 0.02%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master     #348      +/-   ##
============================================
+ Coverage     76.51%   76.54%   +0.02%     
+ Complexity     2241     2240       -1     
============================================
  Files           209      209              
  Lines         10174    10173       -1     
  Branches        906      905       -1     
============================================
+ Hits           7785     7787       +2     
+ Misses         1975     1970       -5     
- Partials        414      416       +2     

Flag	Coverage Δ	Complexity Δ
cli	79.27% <ø> (ø)	0.00 <ø> (ø)
plugin	76.33% <50.00%> (+0.02%)	0.00 <0.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...rch/ad/transport/AnomalyResultTransportAction.java	80.78% <50.00%> (-0.05%)	77.00 <0.00> (ø)
...port/SearchAnomalyDetectorInfoTransportAction.java	59.09% <0.00%> (-4.55%)	4.00% <0.00%> (ø%)
...sticsearch/ad/indices/AnomalyDetectionIndices.java	49.01% <0.00%> (-0.33%)	39.00% <0.00%> (-1.00%)
...pendistroforelasticsearch/ad/NodeStateManager.java	73.52% <0.00%> (+3.92%)	30.00% <0.00%> (+1.00%)
...mazon/opendistroforelasticsearch/ad/NodeState.java	97.77% <0.00%> (+4.44%)	15.00% <0.00%> (+1.00%)