Fix instance variable not defined warnings due to added deprecations

Also fix the `rescue_format` deprecation, it seems it was never being triggered because it was checking for the existence of a not yet defined variable.

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On a side note, I believe tests that rely on [`force_invalid_locale`](https://github.com/carlosantoniodasilva/i18n/blob/3d039354e36005a1829e015dc7bf37f865529a35/test/i18n/exceptions_test.rb#L87-L91) are broken, because the [locale option will never be "invalid"](https://github.com/carlosantoniodasilva/i18n/blob/3d039354e36005a1829e015dc7bf37f865529a35/lib/i18n.rb#L147) since it's going to use the default locale instead, thus no exception is raised.

I can work on a fix for that later if you want, after finishing up a couple things related to I18n deprecations in Rails, just let me know.