Properly check for a pre-existing error in `User#namespace_uniq`

Rails adds an empty error to the Errors object even if you just call
`#[]` on it:

```
[1] pry(main)> u = User.last
[2] pry(main)> u.errors.keys
=> []
[3] pry(main)> u.errors[:username].present?
=> false
[4] pry(main)> u.errors.keys
=> [:username]
```

app/models/user.rb

@@ -372,7 +372,8 @@ class User < ActiveRecord::Base
 
   def namespace_uniq
     # Return early if username already failed the first uniqueness validation
-    return if self.errors[:username].include?('has already been taken')
+    return if self.errors.key?(:username) &&
+      self.errors[:username].include?('has already been taken')
 
     namespace_name = self.username
     existing_namespace = Namespace.by_path(namespace_name)