Fix Bitbucket Server importer error handler
If the Bitbucket Server importer encountered any error during a pull
request import, an undefined local variable or method 'backtrace'
error would previously be thrown.
1ee162b6 dropped a variable that was needed to log a clean backtrace. We add it back now so that error recovery will work again.
Part of #197881 (closed)
Edited by 🤖 GitLab Bot 🤖