Include inner error when printing `TetraError::PlatformError` about tetra

[17cupsofcoffee]

Some background: the reason TetraError generally doesn't include the inner errors in the Display impl is because this doesn't play nicely with error reporting libraries like anyhow or eyre, which use Error::source to print a nice stack with all the nested errors. If you include the source in Display as well as context, you get duplication in the output.

For this reason, the Rust error handling project group recommends that nested errors should either be returned from Display or source, but not both.

The problem is, for PlatformError, InvalidShader, FailedToChangeDisplayMode and TessellationError, I'm not returning them from either 🤦🤦🤦

This is an oversight on my part - those four errors can't return their causes from source (due to the nested types not implementing Error), so I should show them via Display instead.

I will try to fix this issue tomorrow.

As a workaround, if you print the error with Debug, you should be able to see what message is inside it.