I15-1: Check what happens when moving containers fails

When moving containers in the pucks view created in https://github.com/DiamondLightSource/atlas/pull/133 it would be good to get sensible error messages if the operation fails. We should test some common failure modes and make sure it's obvious to the user why they failed:

* Adding a puck to the table in an already occupied position

Review the pucks view introduced in pull request 133 and trace the container-moving flow. Reproduce adding a puck to an already occupied table position, then verify that the operation reports a clear, user-facing reason for failure and that the failure mode is covered by a test.

Written by the indexing model from the issue text.