Fix wrong-type-argument error in helpful-key

[mmarshall540]

Related to #246.

When a key is bound to an anonymous keyboard-macro, helpful-key issues a "wrong type argument" error, as described in the linked issue. For example, if "C-h" is bound to "^?" (to mimic backspace), and one enters "M-x helpful-key C-h", the message will be:

Wrong type argument: symbolp, "^?"

This pull request causes helpful-key to respond with a less confusing error message, like so:

C-h is bound to ^? which is not a command

[Wilfred]

Thanks :)