IMPORT INTO should explicitly return error for unsupported use case

[lance6716]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

IMPORT INTO is a complex statement, it may be executed in multiple TiDB nodes in the cluster. And its internal implementation is different from LOAD DATA which users may think they are similar. So we should explicitly return error for unsupported use case.

For example,

IMPORT INTO t(a, @, c) SET b=(SELECT 'subquery') FROM

will not return error and will not import any data due to an internal panic

2. What did you expect to see? (Required)

error message or data is imported.

3. What did you see instead (Required)

see above

4. What is your TiDB version? (Required)