Add better direction for installing required Debug Bar plugin

[jeffpaul]

If the Sophi Debug Bar plugin is activated without the Debug Bar plugin being active, an admin error is thrown:
Sophi Debug Bar requires Debug Bar plugin

We should look to update this a bit to help direct folks on the next action to take, so something like:
Sophi Debug Bar requires the Debug Bar plugin. Please install and activate that plugin here. -- and hopefully we can link on "here" to something like wp-admin/plugin-install.php?s=debug bar&tab=search&type=term to get them close enough to adding that plugin.