Unexpected error in scans.py also returns a traceback. Refactored pregrader

zesje/pregrader.py

@@ -13,7 +13,7 @@
 import cv2
 import numpy as np
 
-from zesje.database import db, Solution, ProblemWidget, Problem
+from zesje.database import db, Solution, ProblemWidget
 from zesje.images import guess_dpi, get_box, fix_corner_markers
 
 

zesje/scans.py

@@ -54,7 +54,7 @@ def process_pdf(scan_id):
         # TODO: When #182 is implemented, properly separate user-facing
         #       messages (written to DB) from developer-facing messages,
         #       which should be written into the log.
-        write_pdf_status(scan_id, 'error', "Unexpected error: " + traceback.format_exc())
+        write_pdf_status(scan_id, 'error', f"Unexpected error: {error}\n Traceback:\n" + traceback.format_exc())
 
 
 def _process_pdf(scan_id, app_config):