fix(bitbucket-server): PR update with invalid reviewers (#7698)

* fix(bitbucket-server): Do not throw REPOSITORY_CHANGED if API error is about invalid reviewers

Also always log the error in any case for better visibility

* fix: Extract code into helper methods and retry in updatePR

* Fix review feedback

* Update lib/platform/bitbucket-server/index.ts

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

* lint

* Move new helpers into utils.ts and add proper types for error

* Update lib/platform/bitbucket-server/utils.ts

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

Co-authored-by: Rhys Arkins <rhys@arkins.net>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>