reorganize into separate crates

- taskchampion -- core implementation of a replica
 - taskchampion-cli -- command-line interface
 - taskchampion-sync-server -- server implementation (not much yet!)

taskchampion/src/errors.rs

@@ -0,0 +1,10 @@
+use failure::Fail;
+
+#[derive(Debug, Fail, Eq, PartialEq, Clone)]
+pub enum Error {
+    #[fail(display = "Task Database Error: {}", _0)]
+    DBError(String),
+
+    #[fail(display = "Replica Error: {}", _0)]
+    ReplicaError(String),
+}