tree: remove useless "Backend error" message from TreeMergeError

I don't think it adds any contextual information. TreeMergeError is somewhat
similar to BackendError.

cli/tests/test_abandon_command.rs

@@ -317,9 +317,7 @@ fn test_bug_2600_rootcommit_special_case() {
     let stderr = test_env.jj_cmd_internal_error(&repo_path, &["abandon", "base"]);
     insta::assert_snapshot!(stderr, @r###"
     Internal error: Merge failed
-    Caused by:
-    1: Backend error
-    2: The Git backend does not support creating merge commits with the root commit as one of the parents.
+    Caused by: The Git backend does not support creating merge commits with the root commit as one of the parents.
     "###);
 }
 

lib/src/tree.rs

@@ -41,7 +41,7 @@ pub enum TreeMergeError {
         source: std::io::Error,
         file_id: FileId,
     },
-    #[error("Backend error")]
+    #[error(transparent)]
     BackendError(#[from] BackendError),
 }