Better handling of error messages

[ajlamarc]

Right now (esp. on the login screen) there's a lot of indirection / magic values being returned by the auth service's login method, and their usage on the login screen is similarly confusing.

This is generally discouraged - The full error should be thrown or returned and caught by the caller (as Electron permits). This will help us provide more explicit error messages to help with debugging for users.

[drunkplato]

I am considering this in the refactor. Would like to jump on a call to discuss in more detail.