Add a KMessageWidget to report any errors that have occured during
the import:
- Files that have failed to copy into the temporary directory.
- Files that failed to rename/move to their final location.
- Subfolders (as defined by FileRenameFormatter) that failed to create.
Depending on the kind of failure(s), the KMessageWidget will contain
one or two actions to show a detailed list of files and/or subfolder
failures.