Bugs to fix

[Tom Chestnut]

• Need an error to display if user tries to sign up with existing username
• Display an error if user logs in with wrong name/password. Currently an error gets displayed in the log but nothing on the page.
• URL redirects to '/todos' on an incorrect login. Should stay on the original login page.
• Display an error if a todo item is submitted but not all the input fields are completed.