Bug CTHUB - Upload Error Message has content issue

**Describe the Bug**
Improper error messages are being generated for the user when they upload GER data and the system detects cell values that are not in the list of preapproved values.

**Expected Behaviour**
Each error message should only display the corresponding cell value for that error, and not append the previous ones to it.

**Actual Behaviour**
Cell values that are not in the list of acceptable values are being detected during the user upload of GER data. This error detection is correct, but the error message to the user does not appear properly. When multiple cell value errors are detected, each subsequent error appears to append the previous incorrect cell value to the current error message.

**Implications**
It is difficult for the user to properly identify the errors in their sheet, as the messages are not properly describing the errors.

**Steps To Reproduce**
Steps to reproduce the behaviour:
User/Role: IDIR
1. Go to [CTHUB](https://cthub-dev.apps.silver.devops.gov.bc.ca/upload)
2. Select the "Go Electric Rebates Program" to upload a file
3. Upload the 'categories_error_no_warnings.xlsx' file in /Data test files 2025/GER/ that contains data cell content errors
4. See error