[FEATURE] Be more clear on exception for empty request body

[dbwiddis]

Is your feature request related to a problem?

The OpenSearch BaseRestHandler throws an exception if content (the request body) is not consumed (request.content() is never called).

This creates a confusing situation in our code which doesn't require a body such as the provision and status APIs. Our code does throw an exception if there's no body, but the exception message we write is ignored and replaced by the BaseRestHandler's boilerplate.

What solution would you like?

One of the following:

1. Inside the existing hasContent() checks, consume the content (call request.content()) to preserve our own error message, and throw a useful error message.
2. Throw an empty exception without an error message that will be ignored to the existing catch block, and don't read the message there either.
3. Immediately return from the method without bothering with exceptions.

Do you have any additional context?

We do need to check the content existence somehow (as we do), otherwise we'll actually perform the user's requested action but swallow the response message with a useless error.