Expose rate limit errors in public API

Signed-off-by: Vitaliy Potyarkin <v.potyarkin@yadro.com>

acme/api/internal/sender/sender.go

@@ -119,6 +119,14 @@ func (d *Doer) formatUserAgent() string {
 
 func checkError(req *http.Request, resp *http.Response) error {
 	if resp.StatusCode >= http.StatusBadRequest {
+		if resp.StatusCode == http.StatusTooManyRequests {
+			return acme.TooManyRequestsError{
+				StatusCode: resp.StatusCode,
+				Method:     req.Method,
+				URL:        req.URL,
+			}
+		}
+
 		body, err := io.ReadAll(resp.Body)
 		if err != nil {
 			return fmt.Errorf("%d :: %s :: %s :: %w", resp.StatusCode, req.Method, req.URL, err)

acme/errors.go

@@ -2,6 +2,7 @@ package acme
 
 import (
 	"fmt"
+	"net/url"
 )
 
 // Errors types.
@@ -56,3 +57,14 @@ func (p ProblemDetails) Error() string {
 type NonceError struct {
 	*ProblemDetails
 }
+
+// TooManyRequestsError represents API rate limit violations reported by server.
+type TooManyRequestsError struct {
+	StatusCode int
+	Method     string
+	URL        *url.URL
+}
+
+func (e TooManyRequestsError) Error() string {
+	return fmt.Sprintf("too many requests: HTTP %d: %s (%s)", e.StatusCode, e.URL, e.Method)
+}