Throw a more helpful error if `fetch` global does not exist

[kmjennison]

Is your feature request related to a problem? Please describe.

We now support getUserFromCookies in a non-Next.js backend. Next.js provides a fetch polyfill, but other server environments might not, in which case a somewhat confusing "fetch is not defined" error could throw. Related ticket for documentation: #535

Describe the solution you'd like and how you'd implement it

Let's provide a meaningful error message if fetch is not defined when using it in our backend logic.

Is this a breaking change?

No.

Describe alternatives you've considered

None.

[kmjennison]

Closed in #544.