Support multiple exit codes

..with more specific error enums.

taskchampion/src/replica.rs

@@ -113,7 +113,7 @@ impl Replica {
         // check that it already exists; this is a convenience check, as the task may already exist
         // when this Create operation is finally sync'd with operations from other replicas
         if self.taskdb.get_task(uuid)?.is_none() {
-            return Err(Error::DbError(format!("Task {} does not exist", uuid)).into());
+            return Err(Error::Database(format!("Task {} does not exist", uuid)).into());
         }
         self.taskdb.apply(Operation::Delete { uuid })?;
         trace!("task {} deleted", uuid);