improve sliceValidateError.Error performance using switch and strings.Builder (#2765)

fix missing nil pointer check

use simpler switch case

add missing tests

use for-loop instead of range

add benchmark test codes

binding/default_validator_benchmark_test.go

@@ -0,0 +1,20 @@
+package binding
+
+import (
+	"errors"
+	"strconv"
+	"testing"
+)
+
+func BenchmarkSliceValidateError(b *testing.B) {
+	const size int = 100
+	for i := 0; i < b.N; i++ {
+		e := make(sliceValidateError, size)
+		for j := 0; j < size; j++ {
+			e[j] = errors.New(strconv.Itoa(j))
+		}
+		if len(e.Error()) == 0 {
+			b.Errorf("error")
+		}
+	}
+}