luci-app-attendedsysupgrade: avoid redirects, handle redirect errors

When users add an extraneous slash to the server url, then the ASU
server redirects GET requests.  But, this causes an error on POST
requests as the ASU server does not redirect those, so the LuCI
client failed inappropriately with an unhandled error.

Fix both of these by first trimming trailing slashes from the server
url as it is loaded to avoid the redirects.  Then, also handle all
unexpected responses, instead of just a select few.

Links: efahl/owut@724c990d6c
Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>

applications/luci-app-attendedsysupgrade/htdocs/luci-static/resources/view/attendedsysupgrade/overview.js

@@ -338,19 +338,17 @@ return view.extend({
 							}
 						}
 						break;
-					case 400: // bad request
-					case 422: // bad package
-					case 500: // build failed
+					default:  // any error or unexpected responses
 						if (main == true) {
 							poll.remove(this.pollFn);
 							this.handleError(response, data, firmware);
-							break;
 						} else {
 							poll.remove(this.rebuilder_polls[server]);
 							document.getElementById(server).innerText = '🚫 %s'.format(
 								server
 							);
 						}
+						break;
 				}
 			});
 	},
@@ -632,7 +630,7 @@ return view.extend({
 			uci.load('attendedsysupgrade'),
 		]);
 		const data = {
-			url: uci.get_first('attendedsysupgrade', 'server', 'url'),
+			url: uci.get_first('attendedsysupgrade', 'server', 'url').replace(/\/+$/, ''),
 			branch: get_branch(promises[1].release.version),
 			revision: promises[1].release.revision,
 			efi: promises[2],