Checks if a username is valid.

['ValidUsername' = 0, 'AlreadyInUseError' = 1, 'ModerationError' = 2, 'InvalidLengthError' = 3, 'StartsOrEndsWithUnderscoreError' = 4, 'TooManyUnderscoresError' = 5, 'ContainsSpacesError' = 6, 'InvalidCharactersError' = 7, 'ContainsPiiError' = 10, 'ContainsReservedUsernameError' = 12]