Use retryablehttp facilities to retry on read errors

This adds additional copy operations for all requests, which can
decrease performance.  However, in practice network and image decoding
should be slow enough where this does not matter.

Addresses #30

cmd/formats/download/root.go

@@ -1,12 +1,14 @@
 package download
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"image"
 	_ "image/gif"
 	_ "image/jpeg"
 	_ "image/png"
+	"io"
 	"net/http"
 	"time"
 
@@ -31,6 +33,7 @@ func init() {
 	retry.Logger = nil
 	retry.RetryWaitMin = time.Second * 5
 	retry.Backoff = retryablehttp.LinearJitterBackoff
+	retry.CheckRetry = bodyReadableErrorPolicy
 	httpClient = retry.StandardClient()
 	mangadexClient = md.NewClient().WithHTTPClient(httpClient)
 }
@@ -224,3 +227,20 @@ func getImage(client *http.Client, ctx context.Context, url string) (image.Image
 	}
 	return img, nil
 }
+
+func bodyReadableErrorPolicy(ctx context.Context, resp *http.Response, err error) (bool, error) {
+	if retry, err := retryablehttp.DefaultRetryPolicy(ctx, resp, err); retry || err != nil {
+		return retry, err
+	}
+
+	buf := bytes.NewBuffer(nil)
+	_, err = buf.ReadFrom(resp.Body)
+	resp.Body.Close()
+	resp.Body = io.NopCloser(buf)
+
+	if err != nil {
+		return true, nil
+	} else {
+		return false, nil
+	}
+}