Improve Error Handling for Unregistered Account Sign-In

[sumtzehern]

How are you running Flagsmith

• Self Hosted with Docker
• SaaS at flagsmith.com
• Self Hosted with Kubernetes
• Some other way (add details in description below)

Describe the bug

Currently, when a user attempts to sign in with an unregistered account, the application displays a generic "Server Error (500)" message in the toast notification. This error message is not user-friendly and does not provide helpful information to the user.

Steps To Reproduce

1. Go to the sign-in page.
2. Enter an invalid email and password (e.g. foo / bar).
3. Submit the sign-in form.

Expected behavior

The application should display a toast notification with a clear and user-friendly error message, such as "Username provided is not a valid email."

Screenshots

[matthewelwell]

@sumtzehern I am unable to reproduce this behaviour, please can you provide further steps to reproduce this.

[sumtzehern]

This would happen when a user type in a Unregistered username, email would work fine.

[matthewelwell]

Ok, thanks - I've updated the description of the issue with further clarifications regarding this behaviour. If you'd like to submit a PR for this, please feel free to do so. Otherwise, since this isn't a valid use case (we don't support non-email address usernames), this will likely not be prioritised.