Give better error message on non-existant instrument session

[DominicOram]

If I accidentally give a non-existent instrument session when submitting a plan to BlueAPI I get an error like:

RuntimeError: Numtracker error: [{'message': 'Authentication failed', 'locations': [{'line': 3, 'column': 5}], 'path': ['scan']}]

this doesn't really give me any hints to why the authentication has failed.

Acceptance Criteria

• A failure on the above gives a more useful response like "Visit XXX not found"

[tpoliaw]

We don't really know that the visit doesn't exist, only that the user doesn't have access to the visit they've asked for. If the user is not authenticated they won't get as far as numtracker and the request will give a 401.

We could make the error along the lines of Not authorized to create scan number for ixx and cm12345-1. Would that be clearer?

[DominicOram]

Yh, that'll do, thanks