@EntityEmbedDisplay=image plugin: no image-specific warning displays when invalid image selected

Problem/Motivation

when an invalid image is selected within the EntityEmbedDialog, there's no specific warning to that effect. The "image:image" display is simply removed from the list of display plugins, and if it's the only one you receive this error:

This is confusing, and it took me a while to figure to out what is going on.

Steps to recreate the bug manually.

1 Create entity embed button with "File" entity selected and "Image" display plugin selected.

langcode: en
status: true
dependencies:
  module:
    - entity_embed
    - file
    - image
label: 'Image Embed'
id: image_embed
type_id: entity
type_settings:
  entity_type: file
  bundles: {  }
  display_plugins:
    - 'image:image'
  entity_browser: ''
  entity_browser_settings:
    display_review: false
icon_uuid: null
icon_path: null

2) Try to embed through a button in the wysiwyg.

Expected, If a valid image is selected, EntityEmbedDialog moves to the "Embed" step. If an invalid image is selected, an error message is displayed, "This entity is not a valid image. Please select another entity."

Actual, Error appears: "No display options available for the selected entity. Please select another entity."

Steps to recreate the issue in code:

use Drupal\file\Entity\File;    

    file_put_contents('public://llama.jpg', '');
    $image = File::create(['uri' => 'public://llama.jpg']);
    $image->save();

$contexts = [
  'entity' => $image,
  'entity_type' => 'file',
];

$plugins = \Drupal::service('plugin.manager.entity_embed.display')->getDefinitionsForContexts($contexts);

dump($plugins); die();

Expected, various image display plugins should be present, but aren't.

This is despite the admin form having allowed me to select the "image:image" plugin.

Proposed resolution

A more specific message should be displayed when an invalid image is selected when using the ImageFieldFormatter display plugin.

Remaining tasks

- reviews