Improve Error Messaging When Mapping Import Fields

I had created a custom Constituent field with type of Date

When I went to map the field in the import wizard, I was getting an empty drop-down where you would select the field from the CSV.

After consulting with Support, they told me that it was because the CSV did not contain any fields with a valid Date format.

I would have been very helpful if the system would have displayed an error message saying "No fields found in the import file with valid Date format". With this I would have immediately known the issue.

This should be done for each data type.