Two patch files are attached.

+ ses-header-row.patch

  + 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'

  + Using function `user-error' instead of `error' doesn't spawn a
    debug session and backtrace buffer when variable
    `toggle-debug-on-error' is set.

+ ses-user-error.patch

  + Quasi-randomly replaces some calls to function `error' with
    `user-error'.

  + There could be many other desirable points of replacement, but these
    were some obvious ones, and without knowing whether the patch would
    be accepted at all, it's what I invested.


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