Debug: Fix an oops from converting D_JUMP to msgbuf incorrectly.
Raiser: Add missing ErrNoSuchKey string. Whoops.
Add ErrFormat exception, and fix potential overflow in printfmt.

It turns out that printfmt wasn't accounting for a potential low count of the
number of values passed. The check being done was never being triggered
either, making it worse.

Additionally, fixed up err_function grabbing in the vm.
Added isnil keyword function. Fixes #27.