feat(engine): infrastructure

* allow usage of cmn.rs for common types (like Error types)
* instantiate an engine and handle errors, in an initial quick and dirty
  way.

 Fixes #52

src/rust/cli/cmn.rs

@@ -0,0 +1,12 @@
+
+
+#[derive(Debug)]
+pub enum ArgumentError {
+    ConfigurationDirectoryInaccessible(String),
+}
+
+#[derive(Debug)]
+pub struct InvalidOptionsError {
+    pub issues: Vec<ArgumentError>,
+    pub exit_code: i32,
+}