Replace failure by thiserror/anyhow

[gferon]

InternalError was already using thiserror, so this commit finishes transitioning away from failure which means errors can now be better handled in downstream code.

I forgot to mention that the most useful part for me here, is that Error::IoError implements From<std::io::Error> as most stores trait implementations will have to deal with some I/O operations.

TODO:

• [x] Use the new Error in all the store traits
• [x] Prepare MR to libsignal-service-rs at the same time, with all the necessary fixes. Done, see https://github.com/Michael-F-Bryan/libsignal-service-rs/pull/19

[gferon]

@Michael-F-Bryan I think this is ready!