bug#16567: 24.3.50; pcase should signal an error if no case matches

[Stefan Monnier]

> pcase returns nil for this example:
>  (pcase 3
>    (1 1)
>    (2 2))
> it would be more useful if it would signal an error instead.  Manually
> adding a "catch-all case" and inventing some error message is tedious
> and it's also easy to forget.

The "nil default" is also very handy at times and pcase has been in use
for long enough that we can't really change this easily.

But we could add a new pcase-exhaustive macro that does that.


        Stefan