For complex imports, with processing required during the import process, the best practice is to import the spreadsheet into a temporary or staging table and do the processing after the data is in Mendix.  In this way, you can keep track of what records were processed, identify which records and attributes cause problems or have bad data and process in batches if you have a large number of records.

**EDIT**

You're right, importing a large spreadsheet would take a lot of work.  Fortunately, there is a quick way to do this.  For instructions, see this How To: https://docs.mendix.com/howto/tips/import-a-large-excel-file  Once you have the data imported (using the How To, you should be able to get this done in about 15-30 minutes), you can then build a simple microflow to loop through the rows you imported and process them (i.e. convert  dates, number, etc. and use custom error handling so you can identify the specific row that is causing the problem).

Also, as far as I know, Excel importer doesn't have the kind of specific logging you are asking about.  If you look through the stacktrace that is generated when the import fails, you may find some information in there that points to the problem data.  It is likely one of your number/date attributes that cannot be converted because of the data in one of the cells in the spreadsheet.