js: Factor out getResponseError utility function

This function factors out the logic of extracting an error message from
a `Response` object into a reusable utility function.  This will allow
other pages to use the same logic without duplicating it.

js/ajaxUtil.ts

@@ -0,0 +1,16 @@
+export async function getResponseError(r: Response): Promise<string> {
+    let ct = r.headers.get("Content-Type");
+    if (ct && ct.indexOf("json") > -1) {
+        const json = await r.json();
+        if (json.error) {
+            return json.error;
+        }
+    }
+    const html = await r.text();
+    if (html) {
+        const doc = new DOMParser().parseFromString(html, "text/html");
+        return doc.documentElement.textContent ?? "";
+    } else {
+        return r.statusText;
+    }
+}

js/receipt-form.ts

@@ -12,6 +12,7 @@ import CameraInput from "./camera.ts";
 import SlButton from "@shoelace-style/shoelace/dist/components/button/button.js";
 
 import { notify, notifyError } from "./alert";
+import { getResponseError } from "./ajaxUtil.js";
 
 const form = document.forms[0];
 const cameraInput = form.querySelector("camera-input") as CameraInput;
@@ -49,21 +50,8 @@ form.addEventListener("submit", async (evt) => {
         notify("Successfully uploaded receipt", undefined, undefined, null);
         window.location.href = "/receipts";
     } else {
-        let ct = r.headers.get("Content-Type");
-        if (ct && ct.indexOf("json") > -1) {
-            const json = await r.json();
-            if (json.error) {
-                notifyError(json.error);
-                return;
-            }
-        }
-        const html = await r.text();
-        if (html) {
-            const doc = new DOMParser().parseFromString(html, "text/html");
-            notifyError(doc.body.textContent ?? "");
-        } else {
-            notifyError(r.statusText);
-        }
+        const err = await getResponseError(r);
+        notifyError(err);
     }
 });