cli: Use anyhow and thiserror for errors

This patch updates the CLI to use anyhow and thiserror for error
management.  The main feature that our custom `Error` type was providing
was a _list_ of messages, which would allow us to annotate "lower-level"
errors with more contextual information.  This is exactly what's
provided by anyhow's `Context` trait.

(This is setup work for a future PR that will pull the `config` and
`loader` modules out into separate crates; by using `anyhow` we wouldn't
have to deal with a circular dependency between with the new crates.)

cli/Cargo.toml

@@ -20,6 +20,7 @@ harness = false
 
 [dependencies]
 ansi_term = "0.12"
+anyhow = "1.0"
 atty = "0.2"
 cc = "^1.0.58"
 clap = "2.32"