Rework RequestException temporarily as a less breaking change

[Archomeda]

This is a less breaking change that can function as a transition period for the full exception handling rework. Version 2.0 is still a bit off, and this will at least ease the pain of handling Gw2Sharp exceptions via the non-generic RequestException as it was meant to be in the first place, but somehow didn't end up how it was supposed to.

It's a less than ideal change due to the Response property that's now marked as new in the generic RequestException<T>, which means that the non-generic RequestException.Response won't be set (and is hidden) when RequestException<T>.Response is used instead. See the C# documentation.

See #96 for the full change that will happen in v2.0, which includes a deletion of the generic variant of RequestException<T>.

[sonarcloud]

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