Rework HTTP downloader retry logic

Apply retries as global, config-level option `downloadRetries` so that
it can be applied to any aptly command which downloads objects.

Unwrap `errors.Wrap` which is used in downloader.

Unwrap `*url.Error` which should be the actual error returned from the
HTTP client, catch more cases, be more specific around failures.

cmd/mirror_update.go

@@ -41,7 +41,6 @@ func aptlyMirrorUpdate(cmd *commander.Command, args []string) error {
 	}
 
 	ignoreMismatch := context.Flags().Lookup("ignore-checksums").Value.Get().(bool)
-	maxTries := context.Flags().Lookup("max-tries").Value.Get().(int)
 
 	verifier, err := getVerifier(context.Flags())
 	if err != nil {
@@ -54,7 +53,7 @@ func aptlyMirrorUpdate(cmd *commander.Command, args []string) error {
 	}
 
 	context.Progress().Printf("Downloading & parsing package files...\n")
-	err = repo.DownloadPackageIndexes(context.Progress(), context.Downloader(), verifier, context.CollectionFactory(), ignoreMismatch, maxTries)
+	err = repo.DownloadPackageIndexes(context.Progress(), context.Downloader(), verifier, context.CollectionFactory(), ignoreMismatch)
 	if err != nil {
 		return fmt.Errorf("unable to update: %s", err)
 	}
@@ -173,8 +172,7 @@ func aptlyMirrorUpdate(cmd *commander.Command, args []string) error {
 						repo.PackageURL(task.File.DownloadURL()).String(),
 						task.TempDownPath,
 						&task.File.Checksums,
-						ignoreMismatch,
-						maxTries)
+						ignoreMismatch)
 					if e != nil {
 						pushError(e)
 						continue