Made MTTError a proper Error structure.

tests/lib_document_test.rs

@@ -1,6 +1,6 @@
 use morethantext::{
     action::{DocDef, FieldType, Query},
-    MTTError, MoreThanText, Name,
+    ErrorID, MTTError, MoreThanText, Name,
 };
 use uuid::Uuid;
 
@@ -10,10 +10,13 @@ fn errors_on_missing_page() {
     let doc_name = "missing";
     match mtt.get_document(doc_name, "something") {
         Ok(data) => assert!(false, "got '{}', should have been not found error", data),
-        Err(err) => match err {
-            MTTError::DocumentNotFound(result) => assert_eq!(result, doc_name),
-            _ => unreachable!("got {:?}, should have been not found", err),
-        },
+        Err(err) => {
+            let err_id = err.error_id();
+            match err_id {
+                ErrorID::DocumentNotFound => {}
+                _ => unreachable!("got {:?}, should have been not found", err),
+            }
+        }
     }
 }