Remove failure rexports

[mistydemeo]

Based on what I found in studio-lucia/sss_fontbuild#2, it looks like it's not possible to use the failure/failure_derive reexports from the prelude; it seems to be necessary to independently import those crates anyway. As a result, there doesn't seem to really be a reason to reexport them.

I also removed the failure_derive import in this module, since it's not used internally either.

[killercup]

Thanks!

Yeah, failure-derive requires and explicit extern crate failure; to be possible it seems, and that's only the case when you have the crate added to your dependencies. Let's get rid of it.

I think we can use the failure macros, though, if we also re-export the functions they use.

And speaking of failure macros, CI fails with

error: cannot find macro `format_err!` in this scope
  --> examples/thumbnails/src/main.rs:38:24
   |
38 |         .ok_or_else(|| format_err!("couldn't read file name of {:?}", original))?;
   |                        ^^^^^^^^^^ help: you could try the macro: `format_args!`

because we use one of those in the thumbnails guide! :) We could alternatively use with_context here, another thing failure exports which I want to keep.