Fix task cancel exception swallow

[leszekhanusz]

Cancelling a subscription task should generate a CancelledError Exception as explained in issue #544

This PR fixes this problem.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7fb869a) to head (f66008e).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #548   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         2908      2855   -53     
=========================================
- Hits          2908      2855   -53     

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