Better convey duplicate name errors to client (#116)

Motivation: Creating duplicate resources currently results in a 500
error being displayed by the client, with no information about the true
nature of the failure.

Modifications:
* ResourceStore now converts any errors from the etcd client into
  fission errors, capturing the reason for the error in the case of a
  duplicate key (as ErrorNameExists)
* ErrorNameExists errors are signaled to the client with a
  409 (Conflict) HTTP status
* MakeErrorFromHTTP() now reads the body of the error response to
  retrieve the actual error message instead of using the HTTP status
  message
* the controller client now uses MakeErrorFromHTTP() to centralize
  status code -> error code mapping
* tests for all of the resources now check that duplicate resources are
  reported properly

Result: The client can now provide more context when a duplicate name is
given

related to #112

controller/client/client.go

@@ -80,19 +80,7 @@ func (c *Client) url(relativeUrl string) string {
 
 func (c *Client) handleResponse(resp *http.Response) ([]byte, error) {
 	if resp.StatusCode != 200 {
-		var errCode int
-		switch resp.StatusCode {
-		case 403:
-			errCode = fission.ErrorNotAuthorized
-		case 404:
-			errCode = fission.ErrorNotFound
-		case 400:
-			errCode = fission.ErrorInvalidArgument
-		default:
-			errCode = fission.ErrorInternal
-		}
-		return nil, fission.MakeError(errCode,
-			fmt.Sprintf("HTTP error %v", resp.StatusCode))
+		return nil, fission.MakeErrorFromHTTP(resp)
 	}
 	body, err := ioutil.ReadAll(resp.Body)
 	return body, err