[Task](ui): update `SignInForm` default authentication error message, add story

[franzheidl]

Currently, we do not have a dedicated story for a SignInForm with an authentication error in storybook. Also, the default error message that appears when error={true} is passed. "Authentication failed", is too short and clumsy and is missing any punctuation.

New message / wording: "Unable to authenticate the provided credentials."

Tasks

• Implement the new defautl authentication error message in the component.
• Add a story with error={true} to show the default error message
• Update tests as needed