Show sign in errors inline on sign in view

[aerickson14]

What's changed

• Removed the functionality where authentication errors would present an alert

• Added an inline error message to the sign in view for authentication errors
  

• Handle the case where authentication responds with an account locked error
  

How to test

• From an unauthenticated state open up the sign in view either from preferences or the icon in the menu bar
• Fill in your username but provide an incorrect password
• You should see an inline error message "Invalid username and password combination"
• Tapping cancel or next should clear this error message
• If you input a username that is common/short/not an email address like "test" the app will now show a proper error message for account locked. Previously this would generate an alert of the form "Received an unexpected sign in response."

[MattKiazyk]

🎉
So much cleaner - thanks!