ERROR_INDEX isn't seen in the error message

can be discussed

Is there a reason this row number cannot be a part of the error message itself (in addition to diagnostics property)?
It wouldn't be anything new, we already have it for some errors, e.g.

MariaDB [test]> insert into t1 values (1,'foo'),(2,'foobar');

ERROR 1406 (22001): Data too long for column 'c' at row 2

While diagnostics property is useful to detect the number of the flawed row programmatically, in interactive execution having it in the error text is clearly more user-friendly, especially when many values are involved:

MariaDB [test]> insert ignore into t1 values (1,'foo'),(2,'foobar'),(2,'bar'),(3,'a'),(4,'qux'),(4,'quux'),(5,'baz');

Query OK, 5 rows affected, 4 warnings (0.014 sec)

Records: 7 Duplicates: 2 Warnings: 4

MariaDB [test]> show warnings;

+---------+------+----------------------------------------+

| Level | Code | Message |

+---------+------+----------------------------------------+

| Warning | 1265 | Data truncated for column 'c' at row 2 |

| Warning | 1062 | Duplicate entry '2' for key 'PRIMARY' |

| Warning | 1265 | Data truncated for column 'c' at row 6 |

| Warning | 1062 | Duplicate entry '4' for key 'PRIMARY' |

+---------+------+----------------------------------------+

4 rows in set (0.000 sec)