Improve error handling when creating a new site and the user couldn't be created

[ocean90]

There was an error creating the user.

Source: trunk/src/wp-admin/network/site-new.php#L95​

You'll see this message when creating a new site, the email doesn't exist but the domain exists as a username because it's used as a fallback.

I think we should add a username_exists() check before calling wpmu_create_user() here.

[utkarshpatel]

Added check for username exist

[dipesh.kakadiya]

username_exists check added

[utkarshpatel]

move condition before generating pass

[dipesh.kakadiya]

username_exists check added & error message updated

[jeremyfelt]

33804.2.diff uses the text "The domain or path entered conflicts with an existing username."

I'd be okay changing the error message now for clarity. It would be nice to decouple this experience in the future. If we're creating a site and assigning an admin user, we shouldn't have to care about creating a user at the same time.

Related #14215, #33631

[dipesh.kakadiya]

@jeremyfelt

Updated error message has more clarity than previous one.
Thanks for suggestion.

[jeremyfelt]

In 34996:

MS: Show an error if a new site slug conflicts with an existing username.

Props utkarshpatel, dipesh.kakadiya.
Fixes #33804.