Revert "boot.js: better "nil is subclass" message"
Revert "boot.js: better "nil is subclass" message"

This reverts commit 731dad0a1423030c594ad842a861242a7fdaaf6c.

amber-smalltalk/amber,adrian-castravete/amber,adrian-castravete/amber,adrian-castravete/amber,amber-smalltalk/amber,amber-smalltalk/amber