Closed Issue: Add HttpError ctor for both messageDetails and exception [183]

Currently we are using the convention that to return a descriptive messageDetail as well as an exception in HttpError, we create an exception to wrap the inner, just so we have 2 separate exception messages.

Youssef and Ron discussed this offline and settled on the nested exception approach.
HongMei pushed back and recommended the new ctor.

Resolving this issue consists of:
1) Add internal HttpError ctor that takes messageDetails and exception.
2) Add public HttpRequestMessageExtensions.CreateErrorResponse() that takes Message + MessageDetail + Exception.

Comments: We have actually made the properties all settable. So you can set MessageDetails after you construct the HttpError instance.