Enhance error responses for request limit and bring some code from api errors

~~~
mc: <ERROR> Failed to create bucket for URL [http://localhost:9000/newbucket-101]. Reason: [Reduce your request rate.].
~~~

Client sees proper errors now.

pkg/api/quota/errors.go

@@ -0,0 +1,121 @@
+/*
+ * Minimalist Object Storage, (C) 2015 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package quota
+
+import (
+	"bytes"
+	"encoding/xml"
+	"net/http"
+)
+
+// copied from api, no cyclic deps allowed
+
+// Error structure
+type Error struct {
+	Code           string
+	Description    string
+	HTTPStatusCode int
+}
+
+// ErrorResponse - error response format
+type ErrorResponse struct {
+	XMLName   xml.Name `xml:"Error" json:"-"`
+	Code      string
+	Message   string
+	Resource  string
+	RequestID string
+	HostID    string
+}
+
+// Quota standard errors non exhaustive list
+const (
+	RequestTimeTooSkewed = iota
+	BandWidthQuotaExceeded
+	BandWidthInsufficientToProceed
+	SlowDown
+)
+
+// Golang http doesn't implement these
+const (
+	StatusTooManyRequests = 429
+)
+
+func writeErrorResponse(w http.ResponseWriter, req *http.Request, errorType int, resource string) {
+	error := getErrorCode(errorType)
+	errorResponse := getErrorResponse(error, resource)
+	// set headers
+	writeErrorHeaders(w)
+	w.WriteHeader(error.HTTPStatusCode)
+	// write body
+	encodedErrorResponse := encodeErrorResponse(errorResponse)
+	w.Write(encodedErrorResponse)
+}
+
+func writeErrorHeaders(w http.ResponseWriter) {
+	w.Header().Set("Server", "Minio")
+	w.Header().Set("Accept-Ranges", "bytes")
+	w.Header().Set("Content-Type", "application/xml")
+	w.Header().Set("Connection", "close")
+}
+
+// Error code to Error structure map
+var errorCodeResponse = map[int]Error{
+	BandWidthQuotaExceeded: {
+		Code:           "BandwidthQuotaExceeded",
+		Description:    "Bandwidth Quota Exceeded",
+		HTTPStatusCode: StatusTooManyRequests,
+	},
+	BandWidthInsufficientToProceed: {
+		Code:           "BandwidthQuotaWillBeExceeded",
+		Description:    "Bandwidth quota will be exceeded with this request",
+		HTTPStatusCode: StatusTooManyRequests,
+	},
+	SlowDown: {
+		Code:           "SlowDown",
+		Description:    "Reduce your request rate.",
+		HTTPStatusCode: StatusTooManyRequests,
+	},
+}
+
+// Write error response headers
+func encodeErrorResponse(response interface{}) []byte {
+	var bytesBuffer bytes.Buffer
+	encoder := xml.NewEncoder(&bytesBuffer)
+	encoder.Encode(response)
+	return bytesBuffer.Bytes()
+}
+
+// errorCodeError provides errorCode to Error. It returns empty if the code provided is unknown
+func getErrorCode(code int) Error {
+	return errorCodeResponse[code]
+}
+
+// getErrorResponse gets in standard error and resource value and
+// provides a encodable populated response values
+func getErrorResponse(err Error, resource string) ErrorResponse {
+	var data = ErrorResponse{}
+	data.Code = err.Code
+	data.Message = err.Description
+	if resource != "" {
+		data.Resource = resource
+	}
+	// TODO implement this in future
+	data.RequestID = "3L137"
+	data.HostID = "3L137"
+
+	return data
+}