Error messages when adding users could be more informative

[craddm]

:white_check_mark: Checklist

• [x] I have searched open and closed issues for duplicates.
• [x] This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feaxure.
• [x] The feature is still missing in the latest version.
• [x] I have read through the documentation.
• [x] This isn't an open-ended question (open a discussion if it is).

:strawberry: Suggested change

The csv file used here was missing a final field (Country Code) for each user.

(data-safe-haven) deploydsh@0b11d89ae8c5:/workspaces/data-safe-haven$ dsh users add istorm_users.csv
You are logged into the Azure CLI as:                                                                                                                                                                 
        user: Matthew Craddock (1b2a59f2-f3ea-42e7-b8b8-aeea30572764)                                                                                                                                 
        tenant: turing.ac.uk (4395f4a7-e455-4f95-8a9f-1fbaef6384f9)                                                                                                                                   
Are these details correct? [y/n] (y): y
Could not add users from 'istorm_users.csv'.                                                                                                                                                          
Could not add users to Data Safe Haven.  

The error message is not very helpful for pinpointing the error. It could mean anything e.g. couldn't contact Entra, file formatted incorrectly, or elements missing

:steam_locomotive: How could this be done?

It'd be helpful if the error message was more specific - e.g. missing field, missing column, incorrect format, something like that.