Use WTAP_ERR_UNSUPPORTED for input file stuff we can't handle.

WTAP_ERR_UNSUPPORTED_ENCAP means "I can't *write* that particular
encapsulation type to a file of this format", which mainly means "that
file format simply can't handle packets of that type";
WTAP_ERR_UNSUPPORTED means "this file can't currently be supported by
Wireshark, as there's some feature in the file - such as a file or
per-packet encapsulation type - that we don't (yet) handle".

Change-Id: I7977a9eceef9b72d2fd8d2ec7b0bb63a1b62be19
Reviewed-on: https://code.wireshark.org/review/5796
Reviewed-by: Guy Harris <guy@alum.mit.edu>