centralise warnings & error output

[backwardspy]

we just eprintln error: and warning: messages all over the place at the moment. might be nice to collect these up in some standard way so they can be emitted from one place, perhaps with some exit code logic tied to it as well.

would also be a good opportunity to throw a splash of colour onto the output.

[uncenter]

I'm a big fan of https://docs.rs/miette/latest/miette/, we could use that there. Looks pretty and colorful!