Provide feedback if correct API headers are not supplied

When getting started with this API, it took me some debugging to find out that I wasn't setting $headers correctly (i.e. X-Killbill-ApiKey and X-Killbill-ApiSecret headers).  If you dump the response, it's null.

With this patch, you'll get this response:

array(2) {
  ["statusCode"]=>
  int(401)
  ["body"]=>
  string(1541) "




HTTP ERROR 401


Problem accessing /1.0/kb/accounts/7cf19fd6-3f37-4055-8500-ec6549cd79f3. Reason:

    Make sure to set the X-Killbill-ApiKey and X-Killbill-ApiSecret headers
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