Improve Signup component error messages
Currently, e.g. if a user already exists, the error will be something like 'Conflict'
If the validation fails, it would be
We can make those error messages nicer, to say things like 'A user with this username already exists', maybe at this point: https://gitlab.coko.foundation/pubsweet/pubsweet/blob/master/packages/components/Signup/Signup.jsx#L24
Inspired by #104 (closed)