If we don't find a NetScaler signature, don't report an error.

It's *NOT* an error; it's just a file that isn't a NetScaler file.
Otherwise, we report errors on files that should just be passed on to
other open routines.

Also, NetScaler files are *NOT* text files, and we should *NOT* use
".txt" as the suffix.

Change-Id: If001abbbbc3de3ea27439a44a47ce1d6071d38ae
Reviewed-on: https://code.wireshark.org/review/3678
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit b8293f95ad18bc1cc00363627471c880d0f4e5a2)
Reviewed-on: https://code.wireshark.org/review/3679