core: switch to anyhow

This commit adds `anyhow` as a dependency and switches over to it from
Box<dyn Error>.

It actually looks like I've kept all of my errors rather shallow, such
that we don't get a huge benefit from anyhow at present. But now that
anyhow is in use, I expect to use its "context" feature more going
forward.

crates/core/search.rs

@@ -119,7 +119,7 @@ impl SearchWorkerBuilder {
     pub fn preprocessor(
         &mut self,
         cmd: Option<PathBuf>,
-    ) -> crate::Result<&mut SearchWorkerBuilder> {
+    ) -> anyhow::Result<&mut SearchWorkerBuilder> {
         if let Some(ref prog) = cmd {
             let bin = cli::resolve_binary(prog)?;
             self.config.preprocessor = Some(bin);