Modify KiwiStandardResponses#standardErrorResponse to verify the response status is a client or server error

[sleberknight]

KiwiStandardResponses#standardErrorResponse allows any Response.Status to be supplied.

It should validate that the status is either a client (4xx) or server (5xx) error.

Also, remove the javadoc which says "Does not verify that the given status is actually an error status". It can be replaced with an @throws javadoc that says it throws IllegalArgumentException if the given status is not a client or server error.

[sleberknight]

Instead of removing the javadoc, change it to state that the method does verify that the response status is actually an error status code (4xx or 5xx).

[sleberknight]

Also, while there is a small probability that there is some code that calls standardErrorResponse with a status that is not a client or server error code. But this really doesn't make sense, i.e., to call it with a 200 status.