Introduce APIError type and make ErrorEvent.Err public

This makes it a little bit easier to act on API errors that happen while
streaming.

helper.go

@@ -3,12 +3,26 @@ package mastodon
 import (
 	"encoding/base64"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 	"os"
 )
 
+type APIError struct {
+	prefix     string
+	Message    string
+	StatusCode int
+}
+
+func (e *APIError) Error() string {
+	errMsg := fmt.Sprintf("%s: %d %s", e.prefix, e.StatusCode, http.StatusText(e.StatusCode))
+	if e.Message == "" {
+		return errMsg
+	}
+
+	return fmt.Sprintf("%s: %s", errMsg, e.Message)
+}
+
 // Base64EncodeFileName returns the base64 data URI format string of the file with the file name.
 func Base64EncodeFileName(filename string) (string, error) {
 	file, err := os.Open(filename)
@@ -41,15 +55,18 @@ func Base64Encode(file *os.File) (string, error) {
 func String(v string) *string { return &v }
 
 func parseAPIError(prefix string, resp *http.Response) error {
-	errMsg := fmt.Sprintf("%s: %s", prefix, resp.Status)
+	res := APIError{
+		prefix:     prefix,
+		StatusCode: resp.StatusCode,
+	}
 	var e struct {
 		Error string `json:"error"`
 	}
 
 	json.NewDecoder(resp.Body).Decode(&e)
 	if e.Error != "" {
-		errMsg = fmt.Sprintf("%s: %s", errMsg, e.Error)
+		res.Message = e.Error
 	}
 
-	return errors.New(errMsg)
+	return &res
 }