Fix error in ses.el when setting the current row

Fix error in ses.el when setting the current row

* lisp/ses.el (ses-jump, ses-list-local-printers)
(ses-list-named-cells): Use `user-error' for user errors.
(ses-set-header-row): Function `ses-set-header-row' was
determining the current row based upon variable `ses--curcell',
but that variable is NIL until one begins an operation on a cell
(eg. keybindings '=', '"'), so navigating to a row was
insufficient to select that row, and further generated an ERROR
because the code was not expecting a NIL value for variable
`ses--curcell' (bug#47784).