Add an optional arg ERROR-IF-EMPTY to register-read-with-preview

[Leo Liu]

My primary emacs is now 24.3.90 and so I am using more of
register-read-with-preview and noticed one deficiency.

There are two types of commands that use register-read-with-preview:
a) those create registers b) those don't

For b), it seems more desirable to just quit then prompt the user. Thus
I would like to add an optional arg to register-read-with-preview and
fix all its callers accordingly. Comments and how possible it is to fix
it in emacs-24?

Thanks,
Leo