Allow "error" string as log level

[loqimean]

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[anniel-stripe]

We do need LEVEL_ERROR as a level here - otherwise the library will throw an error if the user tries to set Stripe.log_level = Stripe::LEVEL_ERROR. The error message isn't quite correct though - it wasn't changed in #570 where we added the error level.

Right now we technically allow nil, "debug", "info", Stripe::LEVEL_INFO, Stripe::LEVEL_DEBUG, and Stripe::LEVEL_ERROR, but I think "error" should also be a valid value. If you'd like, could you update the string to level conversion and error message to allow for that?