feat: show errors when figuring out the url failed

src/components/misc/api-config.vue

@@ -89,6 +89,7 @@ export default {
 		async setApiUrl() {
 			if (this.apiUrl === '') {
 				// Don't try to check and set an empty url
+				this.errorMsg = this.$t('apiConfig.urlRequired')
 				return
 			}
 
@@ -97,7 +98,7 @@ export default {
 
 				if (url === '') {
 					// If the config setter function could not figure out a url					
-					return
+					throw new Error('URL cannot be empty.')
 				}
 
 				// Set it + save it to local storage to save us the hoops

src/i18n/lang/en.json

@@ -787,7 +787,8 @@
     "change": "change",
     "signInOn": "Sign in to your Vikunja account on {0}",
     "error": "Could not find or use Vikunja installation at \"{domain}\". Please try a different url.",
-    "success": "Using Vikunja installation at \"{domain}\"."
+    "success": "Using Vikunja installation at \"{domain}\".",
+    "urlRequired": "A url is required."
   },
   "loadingError": {
     "failed": "Loading failed, please {0}. If the error persists, please {1}.",