split out separate error type for serve command

src/error.rs

@@ -40,6 +40,16 @@ impl fmt::Display for DisplayWithSources<'_> {
 #[allow(missing_docs)]
 #[derive(Error, Debug)]
 pub(crate) enum Main {
+    #[error(transparent)]
+    ServeCommand(#[from] ServeCommand),
+}
+
+/// Errors returned from the `serve` CLI subcommand.
+// Missing docs are allowed here since that kind of information should be
+// encoded in the error messages themselves anyway.
+#[allow(missing_docs)]
+#[derive(Error, Debug)]
+pub(crate) enum ServeCommand {
     #[error("failed to load configuration")]
     Config(#[from] Config),