Raise error when binding bogus keys

[aadcg]

Start Nyxt with the config file below and notice that C-left won't be bound since the correct way to designate the key is C-left instead of C-<left>.

(define-configuration :search-buffer-mode
  ((keyscheme-map
    (keymaps:define-keyscheme-map "custom" (list :import %slot-value%)
      nyxt/keyscheme:cua
      (list "C-<left>" 'nyxt/mode/search-buffer:search-buffer)))))

[aadcg]

Related to #3584.