Determine required fields even if they're not being mapped

[jonathanstegall]

Is your feature request related to a problem? Please describe.
Sometimes users map object types that have required fields that they aren't mapping. For example, trying to create a Salesforce contact but never using the last name field, which is required.

See this forum post: https://wordpress.org/support/topic/salesforce-ids-not-pulled-back-into-wordpress-sometimes/

Describe the solution you'd like

I wonder if it would be possible to determine all of the required fields, even if they're not being mapped. We could note that they aren't being mapped and that syncing might fail, for example.