Frontend #43 As a user, so I know an email is already in use, I want a message to be displayed on register.

- Conditional rendering
- In `RegistrationPage.js`, use `setState`, `[emailError, setEmailError]` set initially to false.
- Update the catch error to update the state (to true) if an email is already in use.
- In `return` use material ui alerts `<Alerts>` to trigger and display the error message if the useState is set to true. 