client: Have SetCapability() return an error for unsupported caps

[dveeden]

Response to: #1074 (comment)

Example code

package main

import (
	"fmt"

	"github.com/go-mysql-org/go-mysql/client"
	"github.com/go-mysql-org/go-mysql/mysql"
)

func main() {
	conn, err := client.Connect("127.0.0.1:3306", "root", "", "", func(conn *client.Conn) error {
		return conn.SetCapability(mysql.CLIENT_SESSION_TRACK)
	})
	if err != nil {
		panic(err)
	}
	defer conn.Quit()

	res, _ := conn.Execute("SELECT VERSION() AS ver")
	version, _ := res.GetStringByName(0, "ver")
	res.Close()
	fmt.Println(version)
}

Output:

panic: unsupported or unknown capability

goroutine 1 [running]:
main.main()
	/home/dvaneeden/dev/go-mysql-client/main.go:15 +0x1af
exit status 2

Note that we could:

• extract the capability-to-name stuff from CapabilityString() (This is in the other PR, mysql,client: add log for caps we can't set #1074 )
• add the capability name to the error message
• Use slog for structured logging
• Have UnsetCapability() also return an error. But without any real checking for now. Just to keep the signature similar
• Have SetCapability(mysql.CLIENT_PROTOCOL_41) and other base capabilities (opposed to optional capabilities) be accepted without error. Or have an error like "can not set capability that is already set".
• Have SetCapability() return errors when setting optional capabilities that are already set.

[lance6716]

Note that we could:
...

lgtm for current code