Set up a way to get errors from rbspy / pyspy

[petethepig]

Currently we have this setup where we include rust code as a static library with these bindings. All functions return error codes, but no error messages.

We need error messages.

I think the classic C way of doing this is:

• return -1 from the functions
• having some static char *errorMessage variable that is modified every time there's an error.

Maybe there are also better ways, I don't know.

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This is a good issue if you're familiar with C / Rust programming.