refactor(engine): move error, services, document state, event, fonts and Session methods out of lib.rs

crates/engine/src/error.rs

@@ -0,0 +1,20 @@
+#[derive(Debug, thiserror::Error)]
+pub enum EngineError {
+    #[error("unknown command `{0}`")]
+    UnknownCommand(String),
+    #[error("command `{0}` is not available right now: {1}")]
+    Disabled(String, String),
+    #[error("invalid parameters for `{cmd}`: {msg}")]
+    BadParams { cmd: String, msg: String },
+    #[error("no active composition")]
+    NoComp,
+    /// A `comp` reference (name or id) that names no composition.
+    #[error("no composition {0}")]
+    NoSuchComp(String),
+    #[error("{0}")]
+    Project(#[from] effectcraft_project::ProjectError),
+    #[error("{0}")]
+    Other(String),
+}
+
+pub type Result<T> = std::result::Result<T, EngineError>;