fix(router.py): add reason for fallback failure to client-side exception string

make it easier to debug why a fallback failed to occur

litellm/proxy/_new_secret_config.yaml

@@ -1,7 +1,14 @@
 model_list:
-  - model_name: "*"
+  - model_name: "gpt-3.5-turbo"
     litellm_params:
-      model: "*"
+      model: "gpt-3.5-turbo"
+  - model_name: "gpt-4"
+    litellm_params:
+      model: "gpt-4"
+      api_key: "bad_key"
+  - model_name: "gpt-4o"
+    litellm_params:
+      model: "gpt-4o"
 
 litellm_settings:
-  callbacks: ["lakera_prompt_injection"]
+  fallbacks: [{"gpt-3.5-turbo": ["gpt-4", "gpt-4o"]}]