Move main.rs to bin/ and stub some message stuff.

server/src/error.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum ServerError {
+    #[error("notmuch")]
+    NotmuchError(#[from] notmuch::NotmuchError),
+    #[error("flatten")]
+    FlattenError,
+}