Better error message (#4)

client_test.go

@@ -76,6 +76,15 @@ func TestClientDo(t *testing.T) {
 			wantWrappedErr:      errors.New("wrong status code (500 not in [200]): {\"error\":\"some error\"}"),
 		},
 
+		"negative_wrong_status_codes": {
+			method:              http.MethodGet,
+			urlPostfix:          "/health",
+			statusCode:          http.StatusInternalServerError,
+			expectedStatusCodes: []int{http.StatusOK, http.StatusAccepted},
+			respBody:            []byte(`{"error":"some error"}`),
+			wantWrappedErr:      errors.New("wrong status code (500 not in [200, 202]): {\"error\":\"some error\"}"),
+		},
+
 		"negative_not_a_pointer": {
 			method:              http.MethodGet,
 			urlPostfix:          "/health",

error.go

@@ -4,6 +4,8 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strconv"
+	"strings"
 )
 
 // ReqError request error.
@@ -51,8 +53,13 @@ type APIError struct {
 
 // Error implements error interface
 func (e *APIError) Error() string {
-	return fmt.Sprintf("wrong status code (%d not in %v): %s",
-		e.Resp.StatusCode, e.ExpectedStatusCodes, e.Err)
+	codes := make([]string, 0, len(e.ExpectedStatusCodes))
+	for _, code := range e.ExpectedStatusCodes {
+		codes = append(codes, strconv.Itoa(code))
+	}
+
+	return fmt.Sprintf("wrong status code (%d not in [%s]): %s",
+		e.Resp.StatusCode, strings.Join(codes, ", "), e.Err)
 }
 
 // Unwrap provides compatibility for Go 1.13+ error chains.