fix: Generate proper error message for 'unique' validation tag

[jackchenjc]

fixes #966

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PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?)
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?)

Testing Instructions

type User struct {
	Name  string
	Roles []string `validate:"unique"`
}

type Group struct {
	Users []User `validate:"unique=Name"` // only one field can be specified
}

func main() {
	user := User{
		Name:  "admin",
		Roles: []string{"admin", "admin"},
	}

	err := common.Validate(user)
	// User.Roles field should only contain unique elements

	group := Group{
		Users: []User{
			{
				Name: "user1",
			},
			{
				Name: "user1",
			},
		},
	}

	err = common.Validate(group)
	// Group.Users field should only contain unique elements with unique 'Name' values
}

New Dependency Instructions (If applicable)

[cloudxxx8]

LGTM