print background compile errors ASAP (#2596)

reflex/app_module_for_backend.py

@@ -11,7 +11,10 @@ if "app" != constants.CompileVars.APP:
 
 app_module = get_app(reload=False)
 app = getattr(app_module, constants.CompileVars.APP)
-ThreadPoolExecutor(max_workers=1).submit(app.compile_)
+# Force background compile errors to print eagerly
+ThreadPoolExecutor(max_workers=1).submit(app.compile_).add_done_callback(
+    lambda f: f.result()
+)
 
 # ensure only "app" is exposed.
 del app_module