New warning row for non-existent entities (#1946)

* New warning row for non-existent entities

* Update src/panels/lovelace/entity-rows/hui-error-entity-row.ts

* Address my own comments

src/panels/lovelace/entity-rows/hui-error-entity-row.ts

@@ -0,0 +1,39 @@
+import { html, LitElement } from "@polymer/lit-element";
+import { TemplateResult } from "lit-html";
+
+class HuiErrorEntityRow extends LitElement {
+  public entity?: string;
+
+  static get properties() {
+    return {
+      entity: {},
+    };
+  }
+
+  protected render(): TemplateResult {
+    return html`
+      ${this.renderStyle()}
+      Entity not available: ${this.entity || ""}
+    `;
+  }
+
+  private renderStyle(): TemplateResult {
+    return html`
+      <style>
+        :host {
+          display: block;
+          background-color: yellow;
+          padding: 8px;
+        }
+      </style>
+    `;
+  }
+}
+
+declare global {
+  interface HTMLElementTagNameMap {
+    "hui-error-entity-row": HuiErrorEntityRow;
+  }
+}
+
+customElements.define("hui-error-entity-row", HuiErrorEntityRow);