Feat: add unhappy path test for post student

backend/api/main.py

@@ -65,8 +65,20 @@ async def get_tribe(name: str):
 
 @app.post("/students", status_code=status.HTTP_201_CREATED, response_model=StudentModel)
 async def post_student(item: StudentModel):
+    if item.id is not None:
+        return JSONResponse(
+            status_code=status.HTTP_409_CONFLICT,
+            content=f"You can't post a student with an id. It is already registrered. Use PUT to modify it.",
+        )
+
     tribe_name = item.tribe_name
-    tribe = tribe_repo.get(tribe_name)
+    try:
+        tribe = tribe_repo.get(tribe_name)
+    except TribeRepositoryError:
+        return JSONResponse(
+            status_code=status.HTTP_409_CONFLICT,
+            content=f"The tribe {tribe_name} does not exists. You can't add a student in it.",
+        )
 
     student = Student(item.name, tribe)