For now, only one function is provided to signal errors: STk_err. This
function takes two parameters: a C string which constitutes the body of the
message and a Scheme object (a SCM pointer) designating the erroneous object. If the second argument is NIL, it will not be
printed. Execution of the function STk_err never returns. It provokes a
jump at the start of the top-level loop. Hereafter, is a new implementation of
the when function which uses STk_err when given an erroneous
arguments list.