Fix string interpolation in `util.exprimental.decode.bitset` (#2882) (#2883)

* Fix BitSet decoder API when errorBit=False

    When errorBit is set to False, the original code will return
    `Unit` which will be `()` in interpolated string.

* Add testcases for both errorBit cases in BitSetSpec

(cherry picked from commit 42416cb6

)

Co-authored-by: Ocean Shen <30361859+OceanS2000@users.noreply.github.com>