sync: when using --backup-dir don't delete files if we can't set their modtime

This is a problem when syncing a file which just needed its modtime
set with dropbox which can't set the mod time of a file without
re-uploading it.

Before this change we would delete the file, then the server side move
would fail moving the file to the backup-dir because it no longer
existed.

After this change the destination file is moved to the backup-dir
instead of being deleted and the new file is uploaded.

Fixes #2134

backend/dropbox/dropbox.go

@@ -801,7 +801,7 @@ func (o *Object) SetModTime(modTime time.Time) error {
 	// Dropbox doesn't have a way of doing this so returning this
 	// error will cause the file to be deleted first then
 	// re-uploaded to set the time.
-	return fs.ErrorCantSetModTimeWithoutDelete
+	return fs.ErrorCantSetModTime
 }
 
 // Storable returns whether this object is storable

fs/operations/operations.go

@@ -178,9 +178,13 @@ func equal(src fs.ObjectInfo, dst fs.Object, sizeOnly, checkSum bool) bool {
 				return false
 			} else if err == fs.ErrorCantSetModTimeWithoutDelete {
 				fs.Debugf(dst, "src and dst identical but can't set mod time without deleting and re-uploading")
-				err = dst.Remove()
-				if err != nil {
-					fs.Errorf(dst, "failed to delete before re-upload: %v", err)
+				// Remove the file if BackupDir isn't set.  If BackupDir is set we would rather have the old file
+				// put in the BackupDir than deleted which is what will happen if we don't delete it.
+				if fs.Config.BackupDir == "" {
+					err = dst.Remove()
+					if err != nil {
+						fs.Errorf(dst, "failed to delete before re-upload: %v", err)
+					}
 				}
 				return false
 			} else if err != nil {

lib/rest/rest.go

@@ -246,7 +246,12 @@ func (api *Client) Call(opts *Opts) (resp *http.Response, err error) {
 	}
 	if !opts.IgnoreStatus {
 		if resp.StatusCode < 200 || resp.StatusCode > 299 {
-			return resp, api.errorHandler(resp)
+			err = api.errorHandler(resp)
+			if err.Error() == "" {
+				// replace empty errors with something
+				err = errors.Errorf("http error %d: %v", resp.StatusCode, resp.Status)
+			}
+			return resp, err
 		}
 	}
 	if opts.NoResponse {