Add a "check-config" command to validate config files & tests for it

src/error.rs

@@ -45,6 +45,9 @@ pub(crate) enum Main {
 
     #[error("failed to install global default tracing subscriber")]
     SetSubscriber(#[from] tracing::subscriber::SetGlobalDefaultError),
+
+    #[error(transparent)]
+    CheckConfigCommand(#[from] CheckConfigCommand),
 }
 
 /// Errors returned from the `serve` CLI subcommand.
@@ -72,6 +75,16 @@ pub(crate) enum ServeCommand {
     ServerNameChanged(#[from] ServerNameChanged),
 }
 
+/// Errors returned from the `check-config` 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 CheckConfigCommand {
+    #[error("failed to validate configuration")]
+    Config(#[from] Config),
+}
+
 /// Error generated if `server_name` has changed or if checking this failed
 // Missing docs are allowed here since that kind of information should be
 // encoded in the error messages themselves anyway.