patients: return generic error message from patient_file_upload

[davmlaw]

Summary

• Exception detail from patient_file_upload() was being returned directly to the client as {"error": str(e)}
• Changed to return a generic "Upload failed" message; full exception is still logged server-side via log_traceback()

Related: SACGF/variantgrid_private#3829

Test plan

• Upload a valid patient file — confirm success response unchanged
• Trigger an upload failure (e.g. invalid patient ID) — confirm response contains generic error, not exception detail
• Confirm full traceback still appears in server logs