fix: throw if there is no refresh token despite the necessity of refreshing

[yuku]

If OAuth2Client without refresh token has no access token or has an expired access token, OAuth2Client.prototype.getAccessToken should throw an error.

This commit also reduces the number of unreachable code.

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

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[yuku]

I signed it!

[googlebot]

CLAs look good, thanks!

[JustinBeckwith]

Thank you so much for the fix. This was a total 🤦‍♂️ on my part.