feat: add error message parameter to onError callback in subscription

[cromoteca]

Closes #2062.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.52%. Comparing base (dfc0bca) to head (2ff8fe1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2987   +/-   ##
=======================================
  Coverage   92.52%   92.52%           
=======================================
  Files          84       84           
  Lines        3159     3159           
  Branches      770      770           
=======================================
  Hits         2923     2923           
  Misses        184      184           
  Partials       52       52           

Flag	Coverage Δ
unittests	92.52% <100.00%> (ø)

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[taefi]

Generally, this is a nice enhancement, but, isn't it automatically expose some unwanted error messages to the client as well? For instance, when an endpoint method returning a Signal is annotated with @RolesAllowed("ADMIN"), so far, the only thing on client was: "Error in flux ...", and I wonder if this change exposes the the "access denied" error message to the client, and cause a security issue.