[6.x] Improve error handling when requiring starter kits (#14411)

Author: duncanmcclean

src/StarterKits/Installer.php

@@ -244,8 +244,10 @@ protected function prepareRepository(): self
      */
     protected function requireStarterKit(): self
     {
+        $error = null;
+
         spin(
-            function () {
+            function () use (&$error) {
                 $version = $this->branch;
 
                 // Allow dev stability when installing from VCS repo without tagged releases
@@ -265,12 +267,16 @@ function () {
                 try {
                     Composer::withoutQueue()->throwOnFailure()->require($package);
                 } catch (ProcessException $exception) {
-                    $this->rollbackWithError("Error installing starter kit [{$this->package}].", $exception->getMessage());
+                    $error = $exception;
                 }
             },
             "Preparing starter kit [{$this->package}]..."
         );
 
+        if ($error) {
+            $this->rollbackWithError("Error installing starter kit [{$this->package}].", $error->getMessage());
+        }
+
         return $this;
     }