improve error messages, rollback ADT registry when encountering a translation error

Gives a more graceful error message on #9 .

case_studies/tests/src/structs.rs

@@ -56,3 +56,19 @@ impl EvenInt {
         self.add();
     }
 }
+
+
+
+pub struct Foo {}
+
+impl Foo {
+    #[rr::returns("")]
+    pub fn a(&mut self) {
+        assert!(true);
+    }
+
+    #[rr::returns("")]
+    pub fn b(&mut self) {
+        assert!(true);
+    }
+}