Change error message (wp-login.php)

I’d like to change the following error message:

The e-mail could not be sent.
Possible reason: your host may have disabled the mail() function.

The site (channelwiki.org) is using both MediaWiki and WordPress (WordPress is now used for signups and to retrieve lost passwords). This error occurs because old users which signed up via MediaWiki (not WordPress) didn’t have to enter an email address upon sign up. I’d like to change the error to something more user-friendly explaining that this is because no email address is associated with the account and to contact the administrator.

Also, is it possible to remove the stylised WordPress wp-login.php buttons to create a more unified experience so it will be in-keeping with other parts of the site’s design?

Thanks in advance.