refactor(p2p): subscriber.Stop logs but ignores error from unregister topic validator

[renaynay]

This PR was caused by the fact that we now no longer ignore the error from unregister topic validator.

@Wondertan brought up a good point that the API of Subscriber doesn't enforce SetVerifier so it's possible to successfully Start the Subscriber without mounting a validator on it, but Stop would fail due to no validator being present.

This PR reverts the change and logs/ignores the error from unregister topic validator.

[codecov-commenter]

Codecov Report

Merging #127 (ac6f728) into main (a8ce731) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
- Coverage   65.01%   64.97%   -0.05%     
==========================================
  Files          39       39              
  Lines        3276     3275       -1     
==========================================
- Hits         2130     2128       -2     
- Misses        988      989       +1     
  Partials      158      158              

Files	Coverage Δ
p2p/subscriber.go	43.20% <0.00%> (+0.52%)	⬆️

... and 1 file with indirect coverage changes