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

[Helmut Eller]

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.