Bug #98303 Sanity check of ssl-* configuration

Description:
If MySQL is started with:

ssl-ca = /var/lib/mysql/non-existent.pem 
ssl-cert = /var/lib/mysql/server-cert.pem 
ssl-key = /var/lib/mysql/server-key.pem 

while the file `/var/lib/mysql/non-existent.pem` does not exist.

MySQL will start and will log:
```
[Warning] [MY-011302] [Server] Plugin mysqlx reported: 'Failed at SSL configuration: "SSL context is not usable without certificate and private key"'
```

which might be a bit misleading.

When MySQL is started with `mysqlx=0`, it logs:

```
[Warning] [MY-010069] [Server] Failed to set up SSL because of the following SSL library error: SSL_CTX_set_default_verify_paths failed
```

which is a bit more informative, but it still would be nice to include what parts are missing.

By any chances, that's a minor bug.

How to repeat:
-

Suggested fix:
Make the error message a bit more clear.

Hello Iwo P,

Thank you for the report and feedback.

regards,
Umesh