Introduce custom errors for player handling

Custom errors for malformed/nonexistent player IDs as well as a player
not having a high enough balance for the money requested.

This will allow me to encode more information in the errors, for
callers to use.  I'm generally of the opinion that callers should be
checking error conditions before calling these functions, but per
Murphy's law it would be nice to have more information in the error
message.

packages.lisp

@@ -55,6 +55,8 @@
   (:export
    :player
    :players
+   :error-player-nonexistent
+   :error-player-broke
    :player-id :player-balance :player-hand
    :player-exists? :player-bankrupt? :player-can-bet?
    :player-pay