Use WTAP_ERR_DECOMPRESS for decompression errors.

Distringuish "the compression data has a problem" from "the capture file
(not compressed, or after decompression) data has a problem", with
WTAP_ERR_DECOMPRESS used for the former (whether it's the gzipping
decoded by our gunzip code or the Sniffer compression) and
WTAP_ERR_BAD_FILE used for the latter.

Change-Id: I8e6bff7edb480deba00c52a9e5afff607492e085
Reviewed-on: https://code.wireshark.org/review/4568
Reviewed-by: Guy Harris <guy@alum.mit.edu>