Improve configuration error handling of HttpAppender

[Suvrat1629]

This PR introduces improvements to HttpAppender and adds a new test class, HttpAppenderBuilderTest, to enhance test coverage. The changes include:
Updating HttpAppender.java to improve logging behavior.
Adding HttpAppenderBuilderTest.java to verify the builder logic for HttpAppender.
Ensuring that missing configurations (e.g., URL, Layout) correctly log errors.

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (if it fails due to code formatting issues reported by Spotless, simply run ./mvnw spotless:apply and retry)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests for the changes are provided
• Commits are signed (optional, but highly recommended)