Plugin validation fails for required version, settings

[jeremystretch]

Environment

• Python version: 3.6.9
• NetBox version: 2.9.10

Steps to Reproduce

1. Create and enable a plugin with at least one item defined in required_settings.
2. Don't define that setting for the plugin in configuration.py.
3. Run NetBox

Expected Behavior

NetBox should raise an ImproperlyConfigured exception complaining about the missing setting.

Observed Behavior

No exception is raised. This is due a a somewhat strange bug where referencing settings within PluginConfig.validate() suppresses the exception.