[piet-cairo] Provide propper error for strings containing null before passing to cairo

[JAicewizard]

Currently creating something like a piet label containing a null byte as character will crash the program with a vague error and backtrace.

This could be improved by providing a proper error. Ideally cairo-rs should handle this, and I might end up filling an issue on them as well tomorrow.

[cmyr]

This error is definitely a cairo-specific error; the piet text API does not explicitly disallow nul bytes in strings. The piet crate provides a single Error::Platform variant that holds all platform originating errors.

So two thoughts:

• improving the quality of error messages upstream is a good approach. They're receptive to PRs, and so trying to just improve this in cairo-rs would be my first thought.
• The PlatformError does not need to just be us boxing whatever error we get back from cairo; we could, in piet-cairo, define our own error variant that provides more information in some cases; then when we got an error back from cairo-rs we might inspect it and then determine how best to convert it into a piet-cairo error. This is a pragmatic solution; it gives us more control and flexibility, and also allows us to add more context; for instance if an error occurs when trying to create a text layout, we could include the problem string in the error, on our side.

Thinking about this a bit more, I actually think I'm leaning towards the second option? I think the flexibility could be very helpful.