1.0.4.31: remove *internal-error-context*

1.0.4.31: remove *internal-error-context*
 * use nth-interrupt-context to find the context in top-frame instead
   of squirreling it away in *internal-error-context*.
 * moved the defun for nth-interrupt-context up before top-frame.
 * updated NEWS to reflect non-experimental status of x86-64/darwin
   port.