Fix: use a unique temporary file for the error CSV export

[stonebuzz]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes #N/A
• The "Export the log" action wrote the rejected lines to a file named after the uploaded CSV, so two exports using the same file name could overwrite and remove each other's file.
• The export now writes to a unique temporary file in the plugin upload directory and is always downloaded as Errors.csv.

Screenshots (if appropriate):