Improve public APIs to return only module specific errors

[ayeshLK]

Purpose

$subject

Part of #1487

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #291 (a7e2406) into master (f91192f) will decrease coverage by 2.00%.
The diff coverage is 67.39%.

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
- Coverage   88.63%   86.63%   -2.01%     
==========================================
  Files          10       10              
  Lines         528      546      +18     
  Branches      292      306      +14     
==========================================
+ Hits          468      473       +5     
- Misses         60       73      +13     

Impacted Files	Coverage Δ
websub-ballerina/http_service.bal	85.71% <ø> (ø)
websub-ballerina/native.bal	100.00% <ø> (ø)
websub-ballerina/sub_listener.bal	84.50% <63.33%> (-5.82%)	⬇️
websub-ballerina/utils.bal	89.07% <66.66%> (-2.52%)	⬇️
websub-ballerina/resource_discovery.bal	78.78% <100.00%> (-0.38%)	⬇️
websub-ballerina/subscriber_client.bal	85.18% <100.00%> (-1.49%)	⬇️

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