Fix error message when violating key constraint in Kudu insert

When inserting rows in Kudu using Impala, an unfriendly error message is reported from Impala when a unique key constraint is violated. We need to improve this.

impala-shell> insert into t1 values (1,1);
WARNINGS: Error while flushing Kudu session:
Already present: entry already present in memrowset


Error while flushing Kudu session:
Already present: entry already present in memrowset

Another issue is during insert select statements when some inserted rows violate the unique key constraints:

impala-shell> create table t1 (a int, b int) ... <--- kudu table
impala-shell> insert into t1 values(1,1);
impala-shell> insert into t1 select cast(a + 100 as int), b from t1; <-- works well
impala-shell> insert into t1 select cast(a + 100 as int), b from t1;

The last statement reports an error, indicating that the insert failed. However, if we run a select on t1 we can see that some rows were successfully inserted. We should improve the error message and always report the number of inserted rows.