ui_report: Reuse `channel.xhr_error_message` function in `error` method.

We should reuse the `channel.xhr_error_message` in `ui_report` to reduce
code duplication.

Also, I changed the type of `message` parameter to `string` instead of
`string | null` so that we do not need to alter the types of the functions
that depends on the return value of `xhr_error_message`.

web/src/channel.ts

@@ -201,10 +201,7 @@ export function patch(
     return post(options);
 }
 
-export function xhr_error_message(
-    message: string | null,
-    xhr: JQuery.jqXHR<unknown>,
-): string | null {
+export function xhr_error_message(message: string, xhr: JQuery.jqXHR<unknown>): string {
     if (xhr.status.toString().charAt(0) === "4" && xhr.responseJSON?.msg) {
         // Only display the error response for 4XX, where we've crafted
         // a nice response.

web/src/message_edit.js

@@ -1032,7 +1032,7 @@ export function save_message_row_edit($row) {
                 }
 
                 hide_message_edit_spinner($row);
-                const message = channel.xhr_error_message(null, xhr);
+                const message = channel.xhr_error_message("", xhr);
                 const $container = compose_banner.get_compose_banner_container(
                     $row.find("textarea"),
                 );

web/src/ui_report.ts

@@ -1,6 +1,6 @@
 import $ from "jquery";
-import _ from "lodash";
 
+import * as channel from "./channel";
 import * as common from "./common";
 import {$t} from "./i18n";
 
@@ -38,18 +38,8 @@ export function error(
     status_box: JQuery,
     remove_after?: number,
 ): void {
-    if (xhr && xhr.status >= 400 && xhr.status < 500 && xhr.responseJSON?.msg) {
-        // Only display the error response for 4XX, where we've crafted
-        // a nice response.
-        const server_response_html = _.escape(xhr.responseJSON.msg);
-        if (response_html) {
-            response_html += ": " + server_response_html;
-        } else {
-            response_html = server_response_html;
-        }
-    }
-
-    message(response_html, status_box, "alert-error", remove_after);
+    const msg = xhr ? channel.xhr_error_message(response_html, xhr) : response_html;
+    message(msg, status_box, "alert-error", remove_after);
 }
 
 export function client_error(

web/tests/channel.test.js

@@ -348,7 +348,7 @@ test("xhr_error_message", () => {
     msg = "some message";
     assert.equal(channel.xhr_error_message(msg, xhr), "some message: file not found");
 
-    msg = null;
+    msg = "";
     assert.equal(channel.xhr_error_message(msg, xhr), "file not found");
 });