Verify and correct eventual encoding bugs in Tails installer

Originally created by @Anonymous on #10327 (Redmine)

https://labs.riseup.net/code/issues/8556#note-22:

some encoding bugs occured only when run in some non-English locales, when some external process failed and output non-ascii chars in its error message. It’s probably a blocker for uploading to sid, but maybe not for uploading to experimental first.

Related issues

Blocks #8557 (closed)

Edited May 15, 2020 by Anonymous

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information