Fix multisite login issues and error suppression

This PR primaraily fixes an issue where "Users cannot register with Clef" message is displayed to users trying to log in with multisite. 

This error was occurring because when we did `get_users`, we were limiting users to the current multisite site ID. While this *makes sense*, it is different than the normal WordPress paradigm. With normal WordPress login, if you go to any of the multisite's login pages and try to log in, if you are not a member of that site, you will be redirected to the site you are a member of.

This PR removes the blog_id search parameter to the user query and makes Clef conform to the user login pattern found in WordPress core.

In addition, this PR removes login filters which were suppressing errors that our plugin was throwing during login — hooray!