fix: Better handling when no cached token exists, with more decriptive error

[jstlaurent]

Changelogs

• Make sure the ensure_active_token methods will identify that there is no token
• Surface a better error message when it happens, so users can take action to remediate

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Checklist:

• Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
• Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
• Update the API documentation if a new function is added, or an existing one is deleted.
• Write concise and explanatory changelogs above.
• If possible, assign one of the following labels to the PR: feature, fix, chore, documentation or test (or ask a maintainer to do it for you).

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Adresses #251

This bug fix will better handle cases where the OAuth token is None. This might result from a new user that has not logged in, tokens that were not cached, or cached credentials that have
expired. When the client is used in the loader functions, and one such situation occurs, ane exception will be raised that includes a better, actionable error message for the user.

[cwognum]

Thanks, @jstlaurent !

[mercuryseries]

Thanks, @jstlaurent!

[jstlaurent]

Moved the check into the context manager's _enter_ method, so it's more systematic.