## Details
- Use `Result<T, anyhow::Error>`, or equivalently `anyhow::Result<T>`, as the
  return type of any fallible function.
  Within the function, use `?` to easily propagate any error that implements the
  `std::error::Error` trait.
  ```rust
  use anyhow::Result;

  fn get_cluster_info() -> Result<ClusterMap> {
      let config = std::fs::read_to_string("cluster.json")?;
      let map: ClusterMap = serde_json::from_str(&config)?;
      Ok(map)
  }
  ```
- Attach context to help the person troubleshooting the error understand where
  things went wrong. A low-level error like "No such file or directory" can be
  annoying to debug without more context about what higher level step the
  application was in the middle of.
  ```rust
  use anyhow::{Context, Result};
  fn main() -> Result<()> {
      ...
      it.detach().context("failed to detach the important thing")?;
      let content = std::fs::read(path)
          .with_context(|| format!("failed to read instrs from {}", path))?;
      ...
  }
  ```
  ```console
  Error: failed to read instrs from ./path/to/instrs.jsox
  caused by:
      0: No such file or directory (os error 2)
  ```
- Downcasting is supported and can be by value, by shared reference, or by
  mutable reference as needed.
  ```rust
  // If the error was caused by redaction, then return a
  // tombstone instead of the content.
  match root_cause.downcast_ref::<DataStoreError>() {
      Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
      None => Err(error),
  }
  ```
- A backtrace is captured and printed with the error if the underlying error
  type does not already provide its own. In order to see backtraces, the
  `RUST_LIB_BACKTRACE=1` environment variable must be defined.
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