suggestion: better stack trace

[alxndr]

I'm updating my config to use the new API, and must have messed something up... however this stack trace offers no help in where my mistake might be:

Error executing vim.schedule lua callback: vim/keymap.lua:0: Invalid (empty) LHS
stack traceback:
        [C]: in function 'nvim_set_keymap'
        vim/keymap.lua: in function 'set'
        ...ack/paqs/start/which-key.nvim/lua/which-key/mappings.lua:295: in function 'create'
        ...ack/paqs/start/which-key.nvim/lua/which-key/mappings.lua:307: in function 'parse'
        .../pack/paqs/start/which-key.nvim/lua/which-key/config.lua:297: in function 'add'
        .../pack/paqs/start/which-key.nvim/lua/which-key/config.lua:256: in function ''
        vim/_editor.lua: in function <vim/_editor.lua:0>

[alxndr]

(my issue was that the new v3 API is exposed using require("which-key").add() and not .register() — this was not obvious to me from reading :checkhealth which-key)