Better handle invalid rules (error messages from PD)

[morgo]

Consider the following scenario:

tidb> show placement labels;
+--------+----------------+
| Key    | Values         |
+--------+----------------+
| disk   | ["ssd"]        |
| region | ["us-east-1"]  |
| zone   | ["us-east-1a"] |
+--------+----------------+
3 rows in set (0.00 sec)

tidb> CREATE PLACEMENT POLICY p1 PRIMARY_REGION="us-east-1" REGIONS="us-east-1,us-east-2";
Query OK, 0 rows affected (0.08 sec)

tidb> CREATE TABLE t1 (a int) PLACEMENT POLICY=p1;
ERROR 1105 (HY000): failed to notify PD the placement rules: "[PD:placement:ErrRuleContent]invalid rule content, rule 'table_rule_58_1' from rule group 'TiDB_DDL_58' can not match any store"

The syntax is correct, but the rule fails because the region does not exist. I propose the following fixes:

1. We validate PRIMARY_REGION and REGIONS upon creating the placement policy p1 against the list of known values. This won't protect against a race condition that the region could become unavailable in between the policy being created and the table created, so an error could still be returned by PD.

2. If an error is returned by PD, can we make this easier to understand? Something like "'table_rule_58_1' from rule group 'TiDB_DDL_58' can not match any store. Check to make sure that all stores are online, and the constraints you are specifying are able to be set".

3. Can we expose DDL_58 in information_schema.placement_rules? A user might be logging these errors and it should be possible to match internal IDs like this, since we can currently map tables to table IDs.

[sylzd]

/assign