Reorganize the core API (#60)

core/src/error.rs

@@ -0,0 +1,13 @@
+/// An error from the [`crate::Server`] type.
+///
+/// This type represents only circumstances outside the realm of the protocol, and not the specific
+/// results descriebd in the protocol documentation.
+#[derive(Debug, thiserror::Error)]
+pub enum ServerError {
+    /// There is no client with the given ClientId.
+    #[error("No such client")]
+    NoSuchClient,
+
+    #[error(transparent)]
+    Other(#[from] anyhow::Error),
+}