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[Scheme-reports] Return value for unmatched cond-expand

Draft 8 does not say whether the return value for an unmatched cond-expand
is unspecified.

For example, in Chibi,


evaluates to #t.

Meanwhile, the example cond-expand macro on page 72 explicitly tests for no

     (syntax-error "Unfulfilled cond-expand"))

This could be important for portability:  

Some implementations could conceivably not match on any clauses on code
without an else-clause.  I'm just saying that it *could* have adverse
effects, but being explicit about it in the text could fix that.

Should you agree, then perhaps a good place for such a note could be in the
following passage on page 15:

    "[...] Otherwise, the cond-expand has no effect. Unlike cond,
    cond-expand does not depend on the value of any variables."

Christian Stigen Larsen

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