Support multiple exit codes

..with more specific error enums.

taskchampion/src/errors.rs

@@ -1,6 +1,9 @@
 use thiserror::Error;
+
 #[derive(Debug, Error, Eq, PartialEq, Clone)]
+#[non_exhaustive]
+/// Errors returned from taskchampion operations
 pub enum Error {
-    #[error("Task Database Error: {}", _0)]
-    DbError(String),
+    #[error("Task Database Error: {0}")]
+    Database(String),
 }