Variants that fail to start are added to the backend
**Describe the bug**
If you try `agenta variant serve` and it fails because the docker fails to start. The variant is nevertheless added to the UI.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a buggy variant (i.e. missing requirements.txt)
2. Serve the variant
3. Go to the UI

**Expected behavior**
CLI fails and the variant are not added


diff --git a/agenta-backend/agenta_backend/routers/app_variant.py b/agenta-backend/agenta_backend/routers/app_variant.py
index 18d7a4e..0d3841e 100644
--- a/agenta-backend/agenta_backend/routers/app_variant.py
+++ b/agenta-backend/agenta_backend/routers/app_variant.py
@@ -104,6 +104,8 @@ async def start_variant(app_variant: AppVariant) -> URI:
     try:
         return app_manager.start_variant(app_variant)
     except Exception as e:
+        if db_manager.get_variant_from_db(app_variant) is not None:
+            app_manager.remove_app_variant(app_variant)
         raise HTTPException(status_code=500, detail=str(e))
 
 
['agenta-backend/agenta_backend/routers/app_variant.py']