Fix notmuch-describe-key

Fix notmuch-describe-key

Fix notmuch-describe-key crashing for the following two cases
1. format-kbd-macro cannot deal with keys like [(32 . 126)], switch to
use key-description instead.
2. if a function in the current keymap is not bounded, it will crash
the whole process. We check if it is bounded and silently skip it to
avoid crashing.