i15-1: Samples not being present is a warning not an error

When a sample isn't present on a robot load we want to highlight this in the queue as a warning, not an error and then continue on with the rest of the queue as we just assume that the user has set up the samples badly

## Acceptance Criteria
- If the robot fails to load a sample because there was no sample there we should put a warning on the queue item and continue the queue

Start by tracing how a robot sample-load failure is handled and where the queue item is marked as an error. Check the existing queue continuation and warning behavior; done means a missing sample produces a warning and the rest of the queue continues.

Written by the indexing model from the issue text.