Improve http error message to include content

The immediate goal is to be able to gain more diagnostic info when we encounter 502 bad gateway issue and help us investigate further.

I have to fix the test due to exception contract change (we return a derive of HttpRequestExcetion and the exception message also change).  I thinkthis is not a breaking change (or a lesser degree of) since no one that we know of is depending on KuduClient except test.