Return new error `InvalidToken` if user is not yet authenticated

[har0ke]

Description

Return new error InvalidToken if user is not yet authenticated. The library should never panic when the user forgets to authenticate, but return a valid error to the user.

Motivation and Context

Resolves https://github.com/ramsayleung/rspotify/issues/333

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

[ramsayleung]

Thanks for your contribution, this CI tasks failed.

[har0ke]

Okay, I have fixed the formatting.

[ramsayleung]

Thanks for your contribution, it's merged :)