proofreadpage_tests.py: add error text to Exception
proofreadpage_tests.py: add error text to Exception

Add also error text to raised Exception in case of errors during
page.ocr() call.
Main purpose it to facilitate investigations when tests fail.

Bug: T212076
Change-Id: I1c4a4da38b9f6b4f4ec0bdb90bc1e6712eb61630

wikimedia/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core