Definitions might not exist.

Problem/Motivation

The migrations in rocketship_blocks_migration_plugins_alter do not necessarily exist.
Throwing missing index issues when trying to work with migrations.

Steps to reproduce

- enable module rocketship_blocks
- remove the migrateplus.migration.rs_blocks_images.yml (or put in different split)
- try drush migrate:status

Proposed resolution

Add a check to see if the definitions are indeed set.
Did this similar to the checks in rocketship_content.module

Remaining tasks

Review