Hmm. Why don't you try something more like this?
raise [ "err1", "err2", "err3", <etc>][rand(<length>)]
A whole lot simpler, and it would most likely be faster if Ruby had the option to use static arrays >_<
EDIT:
Oh, and BTW, to get RMXP to raise a specific kind of error (instead of just RuntimeError), use
raise <ErrorClass>, "<error message>"
ErrorClass is stuff like EOFError, ArgumentError, NoMethodError, etc