rebase: give a better error message for bogus branch

diff --git a/RelNotes b/RelNotes
index 8fbe7db0e2a61ae4952f611b02d5a61bbcaf2b77..39dc075ee01342db1cd11c47f6de0395df605976 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes-1.5.4.7.txt
\ No newline at end of file
+Documentation/RelNotes/1.7.3.5.txt
\ No newline at end of file