Make plugin load error messages more user-friendly

Users who encounter plugin load errors are usually those who have misconfigured the plugin, rather than the plugin developers.

## Before

The previous error message was more geared towards plugin developers.

```console
$ bundle exec rubocop lib/rubocop.rb
Error: Failed loading plugin `rubocop-performance` because we couldn't determine the corresponding plugin class to instantiate.
First, try upgrading it. If the issue persists, please check with the developer regarding the following points.

RuboCop plugin class names must either be:

  - If the plugin is a gem, defined in the gemspec as `default_lint_roller_plugin'

    spec.metadata['default_lint_roller_plugin'] = 'MyModule::Plugin'

  - Set in YAML as `plugin_class_name'; example:

    plugins:
      - incomplete:
          require_path: my_module/plugin
          plugin_class_name: "MyModule::Plugin"
```

This error message might only confuse the user.

## After

This PR adjusts the message to guide users on the actions they should take when they see the error.

```console
$ bundle exec rubocop path/to/file.rb
Error: Failed to load plugin `rubocop-performance` because the corresponding plugin class could not be determined for instantiation.
Try upgrading it first (e.g., `bundle update rubocop-performance`).
If `rubocop-performance` is not yet a plugin, use `require: rubocop-performance` instead of `plugins: `rubocop-performance` in your configuration.

For further assistance, check with the developer regarding the following points:
https://docs.rubocop.org/rubocop/plugin_migration_guide.html
```

In most cases, this load error occurs because an extension cop doesn't support plugin system yet, `.rubocop.yml` contains `plugins: rubocop-foo`.

In this situation, the user has two options: run `bundle update`, or if that doesn't resolve the issue,
update `.rubocop.yml` to use `require: rubocop-foo` instead.

If contacting the developer is necessary, the migration guide included in this message may be helpful.

Author: koic

lib/rubocop/plugin/load_error.rb

@@ -13,21 +13,12 @@ def initialize(plugin_name)
 
       def message
         <<~MESSAGE
-          Failed loading plugin `#{@plugin_name}` because we couldn't determine the corresponding plugin class to instantiate.
-          First, try upgrading it. If the issue persists, please check with the developer regarding the following points.
+          Failed to load plugin `#{@plugin_name}` because the corresponding plugin class could not be determined for instantiation.
+          Try upgrading it first (e.g., `bundle update #{@plugin_name}`).
+          If `#{@plugin_name}` is not yet a plugin, use `require: #{@plugin_name}` instead of `plugins: `#{@plugin_name}` in your configuration.
 
-          RuboCop plugin class names must either be:
-
-            - If the plugin is a gem, defined in the gemspec as `default_lint_roller_plugin'
-
-              spec.metadata['default_lint_roller_plugin'] = 'MyModule::Plugin'
-
-            - Set in YAML as `plugin_class_name'; example:
-
-              plugins:
-                - incomplete:
-                    require_path: my_module/plugin
-                    plugin_class_name: "MyModule::Plugin"
+          For further assistance, check with the developer regarding the following points:
+          https://docs.rubocop.org/rubocop/plugin_migration_guide.html
         MESSAGE
       end
     end