Prevent orphaned trial accounts

[Nodirbek75]

Update the userLoginUpgrade mutation to return a proper error response if the provided phone number is already linked to another account.

[islandbitcoin]

Based on walkthrough: https://www.canva.com/design/DAG3vRxzgWs/zW9_BjODTnFiBAJ7jX9rKQ/view?utm_content=DAG3vRxzgWs&utm_campaign=designshare&utm_medium=link2&utm_source=uniquelinks&utlId=ha505a314f9

Moving forward with responding with an error message when a TRIAL account attempts to log in with an existing phone number account.

This will set the foundation for the UI (frontend) to trigger user directed flows based on this situation happening

[brh28]

#237