Add more info on "Sequence component does not exist" error

When deploying a sequence that contains a non-existent action (because of a typo for example), the API responds with a 400, containing an error:
```json
{
"code": "befa971cf82a29c21ccda7d5879e2aec",
"error": "Sequence component does not exist."
}
```

I think it would be good if there was more information on there, specifying which action was not found.
This would make debugging much easier, especially for longer sequences and stupid typos.