[CM] C Stack Unwinding

[bil at ccrma.Stanford.EDU]

If you had called s7_error, rather than s7_eval_c_string,
it would have behaved as you expect, but s7_eval interposes
its own set_jmp, so s7_eval_c_string jumps to it.
It might be possible to make these more consistent.
But it's easy to get around this -- just include a
catch in the scheme code, return an error indication
from scheme, and return from bad_func if one is seen.