Improve error messaging when activating multisite users

[bamadesigner]

We had a user on a multisite network try to activate their account and the only error message you get it "Could not create user". This is not helpful. We had to go digging through core to figure out why.

Right now it uses wpmu_create_user() which returns false if wp_create_user() returns an error, instead of returning the error message from wp_create_user(). An easy improvement would be for wpmu_create_user() to return this error message instead.

I've created a patch for a starting point. Will involve discussion over how much we want to update messaging for other interactions that use wpmu_create_user().

[bamadesigner]

Patch for ticket 43601