NEXUS-5472: Added UT for SVN index and fixes

Changes:
* UT for SVN scraper
* fix for all AbstractGeneratedIndexPageScraper descendants: when "unexpected"
  response is hit during dive, scraping should stop, instead continued with next
  Scraper, as the failed one already recognized the remote
* simplified even more, but centralizing IOEx handling
NEXUS-5472: Page UT expanded