nuageinit: Standardize warning/error messages

Standardize the utilities from nuage.lua, to return nil on failure, plus
an error message as a second result, and some value different from nil
on success.

Make warnmsg() and errmsg() append "nuageinit: " by default. Pass an
optional second parameter as false to avoid printing this tag.

Signed-off-by: Jose Luis Duran <jlduran@gmail.com>