checkInput(form_group_el_or_id) {
        if (typeof form_group_el_or_id === 'string') {
            var form_group = document.getElementById(form_group_el_or_id);
        } else {
            var form_group = form_group_el_or_id;
        }
        var input_elements = this.getInputElementsByFormGroup(form_group);
        var input_valid = true;
        var msg = '';
        for (var validator in this.validators) {
            var res = this._checkInputAgainstValidator(input_elements.input, input_elements.label, validator);
            if (res.valid === false) {
                input_valid = false;
                msg = res.msg;
                break;
            }
        }
        return {
            valid: input_valid,
            msg: msg,
            inputElements: input_elements
        };
    }
Check if input is valid or not, return object containing two keys: valid and msg
If input is invalid valid key will be false and msg will contain error message
If input is valid, valid key will be true and msg empty string
@param {string} form_group_el_or_id
@returns {object} valid: boolean, msg: string