Use WTAP_ERR_BAD_FILE for malformed compressed data in DOS Sniffer files.

Get rid of WTAP_ERR_UNC_TRUNCATED and WTAP_ERR_UNC_BAD_OFFSET, and lump
them under WTAP_ERR_BAD_FILE, with an error string; they're just another
form of "this file isn't a valid file of the type in question".

Change-Id: I0e9ac7c2ee66c8d789234a301c1dc2173aef1312
Reviewed-on: https://code.wireshark.org/review/4562
Reviewed-by: Guy Harris <guy@alum.mit.edu>