[Tanzeel0Hussain]

ReverbNation no longer serves the old "Sorry, we couldn't find that page"
text for missing users, so the message check never matched and every
username was reported as claimed (false positive).

Missing users now get HTTP 404 with the page title
"The page you were looking for doesn't exist (404 Not found)". Existing
profiles (e.g. blue) return 200 and do not contain that text.

Changes:

• errorMsg: "Sorry, we couldn't find that page" -> "404 Not found"

Validation (run locally):

• pytest -m validate_targets_fp --chunked-sites "ReverbNation": passed
• pytest -m validate_targets_fn --chunked-sites "ReverbNation": passed
• pytest (default suite): passed
• CLI check with --local: claimed user found, random user not found