Add error module for abstraction

src/error.rs

@@ -0,0 +1,9 @@
+//! Error types.
+
+#[derive(Debug)]
+pub enum Error {
+    InvalidStory(),
+    SourceError(&'static str),
+}
+
+pub type Result<T> = std::result::Result<T, Error>;

src/main.rs

@@ -1,5 +1,6 @@
 //! Main module.
 
+pub mod error;
 pub mod story;
 
 fn main() {