tr: reject unknown character classes during sequence parsing

[can1357]

uutils tr accepts malformed character classes like [:fooclass:] as literal bytes, so invalid input continues processing instead of failing. GNU tr treats unknown class names as a parse error and exits non-zero.

Reproduction Steps

printf 'abc\n' | tr '[:fooclass:]' 'x'
# Expected (GNU): stderr contains "tr: invalid character class 'fooclass'" and exits 1
# Actual (uutils): outputs "xbx" with exit code 0

Impact

This is a fail-open validation bug that breaks GNU-compatible error handling.

---

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/seq/seq-epipe is now passing!