Rename WTAP_ERR_UNSUPPORTED_FILE_TYPE to WTAP_ERR_UNWRITABLE_FILE_TYPE.

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: I22d407efe3ae9fba7aa25f08f050317549866442
Reviewed-on: https://code.wireshark.org/review/5798
Reviewed-by: Guy Harris <guy@alum.mit.edu>