Improve lead error handling and add test coverage

What does this MR do and why?

Resolves #415617 (closed).

Improve lead error handling and add test coverage

• improve overall maintainability

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Setup to simulate SaaS and restart GDK
2. Configure SSO provider (use github or google)
   • set your development gitlab.yml config values - mine looked like the below, setup for github by going on github under developer settings for my profile, redacted my info:

development:
  <<: *base
  omniauth:
    block_auto_created_users: false
    allow_single_sign_on: true
    providers:
    - { name: 'google_oauth2',
        app_id: '',
        app_secret: '',
        args: { access_type: 'offline', approval_prompt: '' } }
    - { name: 'github',
        app_id: 'xxxx',
        app_secret: 'xxxxx',
        args: { scope: "user:email" } }

3. Make sure you don't have a last name set in your GitHub/google account
4. Navigate to /-/trial_registrations/new and sign up for a trial using SSO provider
5. Follow trial registration flow until company form
6. Compare error with screenshot

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #415617 (closed)