bitbucket: Ensure token is valid before proceeding

[chmouel]

Make sure the token is valid before proceeding with the rest of the code.

Fixes #1802

Tested

tested manually

Changes

Submitter Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 65.08%. Comparing base (e4648f2) to head (454c6c0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/provider/bitbucketserver/bitbucketserver.go	76.92%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1803      +/-   ##
==========================================
- Coverage   65.08%   65.08%   -0.01%     
==========================================
  Files         175      175              
  Lines       13473    13481       +8     
==========================================
+ Hits         8769     8774       +5     
- Misses       4118     4120       +2     
- Partials      586      587       +1     

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[zakisk]

/LGTM