added throw error if no account is chosen

[softwarecurator]

This is handy when user doesn't choose an account and cancels the connection so web apps can handle error correctly rather than just a console warning

[nico-herrera]

OMG THIS IS SO NEEDED

[clbrge]

One problem i see with your PR is that we remove warning even when the provider is not compliant. Ideally this whole block should disappear when all provider would upgrade and respect the specs. I would keep the warning and why not throw another error...