Rename WTAP_ERR_UNSUPPORTED_ENCAP to WTAP_ERR_UNWRITABLE_ENCAP.

That makes it clearer what the problem is, and that it should only be
returned by the dump code path, not by the read code path.

Change-Id: Icc5c9cff43be6c073f0467607555fa7138c5d074
Reviewed-on: https://code.wireshark.org/review/5797
Reviewed-by: Guy Harris <guy@alum.mit.edu>