fix: remove panic in internet errors to avoid crashing

[ebreak]

After Casdoor backend throws an error, the front end will show beego panic pages (which is HTML text). SDK can not convert those texts into json, so the SDK will panic an error, which can stop the whole program.
This PR is to ignore internet exceptions in such conditions.

[hsluoyz]

@kininaru return error as 2nd return value, see:

func CasbinJsGetPermissionForUserOld(e IEnforcer, user string) ([]byte, error) {
	policy, err := e.GetImplicitPermissionsForUser(user)
	if err != nil {
		return nil, err
	}
	permission := make(map[string][]string)
	for i := 0; i < len(policy); i++ {
		permission[policy[i][2]] = append(permission[policy[i][2]], policy[i][1])
	}
	b, _ := json.Marshal(permission)
	return b, nil
}

[ebreak]

@hsluoyz done.