basenc: emit partial output on invalid input

[BenWiederhake]

This is a GNU behavior bug (i.e. this is a bug because GNU behaves differently, even though uutils' current behavior could be considered reasonable, too).

Example:

$ echo -n 'ze16jx(mMba' | LC_ALL=C basenc --z85 -d # GNU
missing!basenc: invalid input
[$? = 1]
$ echo -n 'ze16jx(mMba' | cargo run basenc --z85 -d # uutils
basenc: error: invalid input
[$? = 1]

I would prefer to first land #6007 before starting work on this issue.