Better error messages in failure scenarios

[accolasia]

1. If task creation fails because of a conflict... "The task you are trying to create already exists."
2. If resume fails because the task doesn't exist... "The task you are trying to resume is not found."
3. If API-key configuration is invalid... "Unable to connect to server. The API-key configured in Preferences > Idea Flow is invalid."

Anything else you can think of that we've got distinguishable error codes for should have helpful messages.