Service Desk: add dedicated error handler

[Ahuahuachi]

The default AtlassianRestAPI exception handler raises an HTTPError with an empty error message when using the Service Desk API.
The dedicated exception handler attempts to get the error message from the "errorMessage" (singular) property instead of the "errorMessages" (plural) property

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 34.35%. Comparing base (795705b) to head (b88058e).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
atlassian/service_desk.py	16.66%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1434      +/-   ##
==========================================
- Coverage   34.38%   34.35%   -0.03%     
==========================================
  Files          46       46              
  Lines        8586     8598      +12     
  Branches     1601     1603       +2     
==========================================
+ Hits         2952     2954       +2     
- Misses       5511     5519       +8     
- Partials      123      125       +2     

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