Replace invalid unicode with U+FFFD in `task import`

The `task export` from 2.6.2 just sort of blasts out the bytes it finds in `*.data`, regardless of whether those are valid utf-8 (and thus valid JSON). We can't change behavior of released code, but we can make `task import` a bit more forgiving, and translate those to U+FFFD (�) in the imported tasks, rather than crashing.