Waiver webhook failed when user record exists without role

[taesungh]

We had a few instances of the waiver webhook endpoint returning status 400 because some volunteers/mentors/judges had previously logged into the site but did not apply so did not have a role attribute causing the validation for UserRecord to fail. We could maybe add a "guest" role to all users when logging in, but in practice, non-hackers would have been added to the database before signing the waiver, so this is not a huge issue. We should at least properly handle ValidationError instead of letting that surface.