Exception is now thrown if migration_dependencies is incorrectly formatted

Formatting errors in the migration_dependencies property of a migration now throws an InvalidPluginDefinitionException.

Before:
Incorrectly formatted migration_dependencies, such as when migrations are listed without an optional or required key, would throw a TypeError which did not clearly specify which migration contained the issue.

migration_dependencies:
  - migration_1

After:
Incorrectly formatted migration_dependencies now throws an InvalidPluginDefinitionException with a message telling the user which migration contains the issue.

Examples of correctly formatted migration_dependencies are:

• Non-present (ie migration_dependencies is not set)
• Empty:
  migration_dependencies: {}
• Contain either optional or required keys, or both:
  

  migration_dependencies:
    optional:
      - migration_1
  

  migration_dependencies:
    optional:
      - migration_1
    required:
      - migration_2
  

This only requires action from module developers if they are using incorrectly formatted migration_dependencies.