Improve error message on SPATIAL types/indexes not supported

Bug Report

You can improve the parser to say this feature is not supported or a more specific error message like "SPATIAL index is not supported yet".

1. Minimal reproduce step

create table ti (pk int primary key, p point not null SRID 0, spatial index (p)) charset utf8mb4 engine=innodb;

2. What did you expect to see?

No error in MySQL

3. What did you see instead

mysql> create table ti (pk int primary key, p point not null SRID 0, spatial index (p)) charset utf8mb4 engine=innodb;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 44 near "point not null SRID 0, spatial index (p)) charset utf8mb4 engine=innodb" 

4. What is your TiDB version?

+-------------------------+--------------------------------------------------------------------------+
| Variable_name           | Value                                                                    |
+-------------------------+--------------------------------------------------------------------------+
| innodb_version          | 5.6.25                                                                   |
| protocol_version        | 10                                                                       |
| tidb_analyze_version    | 2                                                                        |
| tidb_row_format_version | 2                                                                        |
| tls_version             | TLSv1,TLSv1.1,TLSv1.2                                                    |
| version                 | 5.7.25-TiDB-v5.2.2                                                       |
| version_comment         | TiDB Server (Apache License 2.0) Community Edition, MySQL 5.7 compatible |
| version_compile_machine | x86_64                                                                   |
| version_compile_os      | osx10.8                                                                  |
+-------------------------+--------------------------------------------------------------------------+