Wrong warning message during foreign key creation failure (not compatible charset)

[Arturas Moskvinas]

Percona Server 5.6 report misleading warning when foreign key creation fails due to incompatible foreign key character set.

Reproduction:
test> create table a(id varchar(1) character set utf8mb4, primary key(id));
test> create table b(id varchar(1) character set utf8, b_id varchar(1) character set utf8, primary key(id), CONSTRAINT a_fkey FOREIGN KEY (b_id) REFERENCES a (id));
ERROR 1215 (HY000): Cannot add foreign key constraint
test> show warnings
```
root@perconatraindb03-sjc1 test> show warnings;
+---------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 150 | Create table 'test/b' with foreign key constraint failed. There is no index in the referenced table where the referenced columns appear as the first columns.
 |
| Error | 1215 | Cannot add foreign key constraint |
+---------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)
```

Expected:
Something more meaningful than column not appearing in index a first column.

[Arturas Moskvinas (arturas-w)]

Tested on Percona server 5.6.36

[Sveta Smirnova (svetasmirnova)]

Thank you for the reasonable feature request.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-1811