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 Bad Request
.
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)