How to display message with ngTagsInput?

Question

                <div class="form-group">
                    <label for="inputAlertEmails" class="col-sm-2 control-label">Alert Emails</label>
                    <div class="col-sm-8">
                        <tags-input type="text"
                                    id="inputAlertEmails" ng-model="ctrl.inputAlertEmails"
                                    min-tags="1" max-tags="5"
                                    on-tag-added="ctrl.UpdateAlertEmails()"
                                    on-tag-removed="ctrl.UpdateAlertEmails()"
                                    placeholder="Enter comma separated valid email ID's to get alerts for your service."
                                    uib-tooltip="Please make sure you confirm the subscription sent by AWS on all email ID entered here."
                                    min-length="5" allowed-tags-pattern="[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,4}$"></tags-input>
                    </div>
                </div>

The above field in the form takes as input multiple email-ID's. I want to add the validation here that whenever the field is touched, but there is no email-ID, an error message should be displayed besides the box in the form which says "please enter an email ID". I have changed min-tags-input to 1 but it doesn't display an error message and required tag doesn't work.