Detect when Protobuf streams start with other file types' magic numbers

[adamnovak]

If you try and turn a graph in any format other than vg Protobuf into JSON in the streaming mode, we just open the stream and try to read Graph objects. If this is actually not anything with VPKG framing but just a HashedGraph or PackedGraph or something, we don't look at the magic number and we fail to parse it as Protobuf and crash out.

vg/src/subcommand/view_main.cpp

Lines 516 to 518 in c9e7e2b

	get_input_file(file_name, [&](istream& in) {
	vg::io::for_each(in, lambda);
	});

We should assume the user will put square pegs into all round holes and notice and help them out.