legacy (.bak) import: If backup files can't be opened, just try one that couldn't be opened

fix https://github.com/deltachat/deltachat-android/issues/1768

src/dc_tools.rs

@@ -514,6 +514,7 @@ pub(crate) async fn get_next_backup_path(
 ) -> Result<(PathBuf, PathBuf), Error> {
     let folder = PathBuf::from(folder.as_ref());
     let stem = chrono::NaiveDateTime::from_timestamp(backup_time, 0)
+        // Don't change this file name format, in has_backup() we use string comparison to determine which backup is newer:
         .format("delta-chat-backup-%Y-%m-%d")
         .to_string();
 

src/imex.rs

@@ -179,10 +179,15 @@ pub async fn has_backup_old(context: &Context, dir_name: impl AsRef<Path>) -> Re
                         info!(context, "backup_time of {} is {}", name, curr_backup_time);
                         sql.close().await;
                     }
-                    Err(e) => warn!(
-                        context,
-                        "Found backup file {} which could not be opened: {}", name, e
-                    ),
+                    Err(e) => {
+                        warn!(
+                            context,
+                            "Found backup file {} which could not be opened: {}", name, e
+                        );
+                        if newest_backup_path.is_none() {
+                            newest_backup_path = Some(path); // If there is no other backup file, just try this one
+                        }
+                    }
                 }
             }
         }