Enhancement: Newsletter subscrption import failed emails

### Impacted plugin

Jetpack

### What

Improve the import failed email to be more actionable

### How

Currently we don't provide much info then a import fails but ask the user to contact support.

We should tailor the message based on what happend.

See https://github.com/Automattic/jpop-issues/issues/9032

Begin by locating Jetpack's newsletter subscription import failure flow and review the linked issue 9032 for the expected failure cases. Define what information each failure email should provide and consider the work complete when users receive actionable, failure-specific guidance instead of only being told to contact support.

Written by the indexing model from the issue text.