Test atomic PUTs, and make them work.

Improve apiserver/logger.go's interface (it's pretty cool now).

Improve apiserver's error reporting to clients.

Improve client's handling of errors from apiserver.

Make failed PUTs return 409 (conflict)-- http status codes are amazingly
well defined for what we're doing!

pkg/client/client.go

@@ -100,16 +100,27 @@ func (c *Client) doRequest(request *http.Request) ([]byte, error) {
 	if err != nil {
 		return body, err
 	}
-	if response.StatusCode < http.StatusOK || response.StatusCode > http.StatusPartialContent {
+
+	// Did the server give us a status response?
+	isStatusResponse := false
+	var status api.Status
+	if err := api.DecodeInto(body, &status); err == nil && status.Status != "" {
+		isStatusResponse = true
+	}
+
+	switch {
+	case response.StatusCode == http.StatusConflict:
+		// Return error given by server, if there was one.
+		if isStatusResponse {
+			return nil, &StatusErr{status}
+		}
+		fallthrough
+	case response.StatusCode < http.StatusOK || response.StatusCode > http.StatusPartialContent:
 		return nil, fmt.Errorf("request [%#v] failed (%d) %s: %s", request, response.StatusCode, response.Status, string(body))
 	}
 
 	// If the server gave us a status back, look at what it was.
-	var status api.Status
-	if err := api.DecodeInto(body, &status); err == nil && status.Status != "" {
-		if status.Status == api.StatusSuccess {
-			return body, nil
-		}
+	if isStatusResponse && status.Status != api.StatusSuccess {
 		// "Working" requests need to be handled specially.
 		// "Failed" requests are clearly just an error and it makes sense to return them as such.
 		return nil, &StatusErr{status}