Print error and exit instead of diplaying an empty map for unsupported map types

[qmonnet]

See discussion in https://github.com/libbpf/bpftool/issues/54.

No need to display an empty maps for types for which we know we can't display anything - apparently the BPF ring buffer, likely the perf event array maps too, maybe a few others. Bpftool should just tell it cannot dump these types.