When moving by sexp (C-M-f, C-M-u and so on) and point is already at a boundary preventing further movement, Emacs currently signals an internal error such as

 Scan error: "Containing expression ends prematurely", 5010, 5010

or

 Scan error: "Unbalanced parentheses", 5010, 1

which is unhelpful and rather looks as if something went wrong in the internal machinery.

The attached patch does away with this error when the commands are invoked interactively; programmatic use of the functions will get the scan-error just like before. There didn't seem to be much point in replacing the errors with new messages so the current version of the patch doesn't.