Use private class attributes for error messages

1 job for refactor in 13 minutes and 29 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Test
failed #393754
tests

00:13:29

 
Name Stage Failure
failed
tests Test
ERROR tests/test_surface.py
!!!!!!!!!!!!!!!!!!! Interrupted: 3 errors during collection !!!!!!!!!!!!!!!!!!!!
============================== 3 errors in 33.28s ==============================
Uploading artifacts for failed job
Uploading artifacts...
WARNING: coverage.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1