Make load_cap_file() just return a success/fail indication.

We don't care *why* it fails; it's printed a message to indicate that.
Just have it indicate whether it succeeded or not.

Change-Id: I98160999baa5f25ea03ef803b7dbf77dff1518de
Reviewed-on: https://code.wireshark.org/review/4213
Reviewed-by: Guy Harris <guy@alum.mit.edu>