report correct PDF page numbers

Previously we were using zero-based indexing, which is confusing.

zesje/helpers/pdf_helper.py

@@ -85,9 +85,10 @@ def process_pdf(pdf_id):
             report_error('PDF is not from this exam')
             return
 
-        # Process individual pages
+        # Process individual pages, ensuring we report the page numbers
+        # starting from 1.
         failures = []
-        for i, (image, qr) in enumerate(zip(images, extracted_qrs)):
+        for i, (image, qr) in enumerate(1, zip(images, extracted_qrs)):
             report_progress(f'Processing page {i} / {len(images)}')
             if qr is None:
                 failures.append(image)