internal/mcp/jsonschema: generalize error wrapping

Add a wrapf function to simplify defers that wrap errors.

Change-Id: If4bedec7d3f56f609d3ca4620aebe7637a59f6bf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

jsonschema/validate.go

@@ -42,11 +42,7 @@ type state struct {
 
 // validate validates the reflected value of the instance.
 func (st *state) validate(instance reflect.Value, schema *Schema, callerAnns *annotations) (err error) {
-	defer func() {
-		if err != nil {
-			err = fmt.Errorf("%s: %w", schema, err)
-		}
-	}()
+	defer wrapf(&err, "validating %s", schema)
 
 	st.stack = append(st.stack, schema) // push
 	defer func() {