Cosmetic changes to MvL's change to unichr():

- the correct range for the error message is range(0x110000);

- put the 4-byte Unicode-size code inside the same else branch as the
  2-byte code, rather generating unreachable code in the 2-byte case.

- Don't hide the 'else' behine the '}'.

(I would prefer that in 4-byte mode, any value should be accepted, but
reasonable people can argue about that, so I'll put that off.)