Update exitError and initError methods to have valid BodyPublishers.

[briensea]

Issue #, if available: The current use of HttpRequest.newBuilder() for post requests in initError and exitError will result in a NullPointerException.

Description of changes:

This change rectifies this issue by doing the following:

• Providing a non null BodyPublisher to the POST() method call.
• This BodyPublisher contains the ErrorRequest JSON object as required by the Lambda Extensions API
• This is acheived through the help of a utility method to reduce code repition among exitError and initError method calls.
• Updates Javadoc comments to account for new required method parameters.
• In addition to this , the change also updates comment terminology to align with the Lambda Extensions API

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