Move code into cli directory

cli/src/error.rs

@@ -0,0 +1,24 @@
+#[derive(Debug)]
+pub struct Error(pub String);
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+impl Error {
+    pub fn grammar(message: &str) -> Self {
+        Error(format!("Grammar error: {}", message))
+    }
+
+    pub fn regex(message: &str) -> Self {
+        Error(format!("Regex error: {}", message))
+    }
+
+    pub fn undefined_symbol(name: &str) -> Self {
+        Error(format!("Undefined symbol `{}`", name))
+    }
+}
+
+impl From<serde_json::Error> for Error {
+    fn from(error: serde_json::Error) -> Self {
+        Error(error.to_string())
+    }
+}