bevy_render: Derive Error on ViewportConversionError

[MrGVSV]

Objective

The ViewportConversionError error type does not implement Error, making it incompatible with BevyError.

Solution

Derive Error for ViewportConversionError.

I chose to use thiserror since it's already a dependency, but do let me know if we should be preferring derive_more.

Testing

You can test this by trying to compile the following:

let error: BevyError = ViewportConversionError::InvalidData.into();