add load data fail log

[xiongjiwei]

What problem does this PR solve?

add log when loading data with errors

a new session variable tidb_load_broken_data= on (default) | off.
if tidb_load_broken_data is on, load data will try to fix broken data in the file and insert it into the table.
if tidb_load_broken_data is off, load data will ignore broken data. (PBOC use)

table schema

create table t(a int, b int);

data in file

1,2
2,3,,
1,
a,2

1,3

load data log

time="2021-01-19T14:41:41+08:00" level=info msg="load file [tidb.out], line 2: [2,3,,] has too much columns"
time="2021-01-19T14:41:41+08:00" level=info msg="load file [tidb.out], line 3: [1,] [types:1292]Truncated incorrect FLOAT value: ''"
time="2021-01-19T14:41:41+08:00" level=info msg="load file [tidb.out], line 4: [a,2] [types:1292]Truncated incorrect FLOAT value: 'a'"
time="2021-01-19T14:41:41+08:00" level=info msg="load file [tidb.out], line 5: [] is missing columns"

if tidb_load_broken_data=on

mysql root@127.0.0.1:test> select * from t;
+---+--------+
| a | b      |
+---+--------+
| 1 | 2      |
| 2 | 3      |
| 1 | 0      |
| 0 | 2      |
| 0 | <null> |
| 1 | 3      |
+---+--------+
6 rows in set

if tidb_load_broken_data=off

mysql root@127.0.0.1:test> select * from t;
+---+---+
| a | b |
+---+---+
| 1 | 2 |
| 1 | 3 |
+---+---+
2 rows in set

[xiongjiwei]

@bb7133 PTAL

[bb7133]

LGTM