Show explicit message to the user when API token is expired (check status 401 or 403 when calling API)

[aliceboizet]

If the token is expired in BMS when we load the shiny app, then the loading data fails telling "No data found for study..."
We should check the response.status when loading data and tell the user "Unable to load data, you may need to log in again to [apiURL]".
Check this also for every call made in the apps.

[aliceboizet]

Create a function in utils : handle_api_response(resp) that throws error depending on resp$status_code.
Use this function after each brapi call in a tryCatch to notify with the thrown error.