[FIX] Weblinkchecker: Report malformed URLs (port)

Don't throw URL exception in the checker thread if the URL cannot be
parsed.

Introduce NotAnURLError exception to allow information about malformed
URLs to be passed to the reporting facility.

This is ported from 405b73862c2859f816dcfe0313c46bde760f82f3 in compat.

Bug: T76294
Change-Id: I0474ddad3b19faed598cf14c647e3c62e9270f50