SC Account Creation - Doesn't validate if username already exists

[TimCliff]

If I go to https://steemconnect.com/accounts/create and try to create a new account with a username that is already taken, there is no feedback to the user that the username cannot be used. It even lets me sign the transaction to create the account and doesn't give an error message after the transaction is signed to indicate it failed.

[TimCliff]

I created a separate issue (https://github.com/steemit/steemconnect/issues/351) to deal with the fact that no error message was displayed.