UX: show a generic error on upload for XHR status 0 (#10521)

This indication covers all cases of network errors, not just "cancelled by user".
The post upload component already has its own handling for user-cancelled uploads, but the generic upload component does not.

Tested by stopping my localhost server right before attempting to upload a file.

app/assets/javascripts/discourse/app/lib/uploads.js

@@ -269,8 +269,9 @@ export function getUploadMarkdown(upload) {
 export function displayErrorForUpload(data, siteSettings) {
   if (data.jqXHR) {
     switch (data.jqXHR.status) {
-      // cancelled by the user
+      // didn't get headers from server, or browser refuses to tell us
       case 0:
+        bootbox.alert(I18n.t("post.errors.upload"));
         return;
 
       // entity too large, usually returned from the web server

app/assets/javascripts/discourse/app/mixins/upload.js

@@ -106,7 +106,7 @@ export default Mixin.create({
     });
 
     $upload.on("fileuploadfail", (e, data) => {
-      if (!data || data.errorThrown !== "abort") {
+      if (!data) {
         displayErrorForUpload(data, this.siteSettings);
       }
       reset();