rebase: give a better error message for bogus branch

diff --git a/RelNotes b/RelNotes
index 1cb2571eaae44b1e01fa4713c6c4b147eed9ac4a..39dc075ee01342db1cd11c47f6de0395df605976 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes-1.6.5.2.txt
\ No newline at end of file
+Documentation/RelNotes/1.7.3.5.txt
\ No newline at end of file