When MailPoet has a form with a required custom field, our connector fails silenty

[JurriaanK]

When a MailPoet List has a Form with a required custom field attached to it, like this:

Our connector fails to add subscribers to a list because of a MailPoet API error:
2025-12-16 14:15:42.989475 - DEBUG --> GFFeedAddOn::maybe_process_feed(): Checking for feeds to process for entry #2306.
2025-12-16 14:15:42.989789 - DEBUG --> GFFeedAddOn::pre_process_feeds(): Found 1 feeds for form #11.
2025-12-16 14:15:42.990710 - DEBUG --> GFFeedAddOn::maybe_process_feed(): Starting to process feed (#3 - ) for entry #2306.
2025-12-16 14:15:42.995649 - ERROR --> GravityWP\Connector_MailPoet_GravityForms\GravityWP_Connector_MailPoet_GravityForms::process_feed(): MailPoet API Error: Missing value for custom field "Click Here for Offer"
2025-12-16 14:15:42.997931 - DEBUG --> GFFeedAddOn::fulfill_entry(): Entry #2306 marked as fulfilled.

For the free version of out plugin I don't want to support custom fields, but we can improve the feedback for the user:

1. Add a warning to the Feed settings that required custom fields prevents subscribers from being added using the connector. If possible detect if there are custom fields
2. Add an entry note on success or error while adding the subscriber.