Switch from failure to anyhow

[alexcrichton]

This commit switches all of wasm-bindgen from the failure crate to
anyhow. The anyhow crate should serve all the purposes that we
previously used failure for but has a few advantages:

• It's based on the standard Error trait rather than a custom Fail
  trait, improving ecosystem compatibility.
• We don't need a #[derive(Fail)], which means that's less code to
  compile for wasm-bindgen. This notably helps the compile time of
  web-sys itself.
• Using Result<()> in fn main with anyhow::Error produces
  human-readable output, so we can use that natively.