Update error strings so BP Groupblog's custom blog validation works.

When WP merged multisite functionality into core, they changed a bunch of
strings from 'blog' to 'site'.

BP Groupblog's custom blog validation relies on some of these strings when
parsing the error message.

Since the error message strings changed, BP Groupblog's validation could
never properly validate.

This commit fixes this issue, which occurred in WP 3.0 and 3.3.1.

We should watch out for any string changes in the future to prevent this
from happening again.