tools: ynl: print some information about attribute we can't parse

tools: ynl: print some information about attribute we can't parse

When parsing throws an exception one often has to figure out which
attribute couldn't be parsed from first principles. For families
with large message parsing trees like rtnetlink guessing the
attribute can be hard.

Print a bit of information as the exception travels out, e.g.:

  # when dumping rt links
  Error decoding 'flags' from 'linkinfo-ip6tnl-attrs'
  Error decoding 'data' from 'linkinfo-attrs'
  Error decoding 'linkinfo' from 'link-attrs'
  Traceback (most recent call last):
    File "/home/kicinski/linux/./tools/net/ynl/cli.py", line 119, in <module>
      main()
    File "/home/kicinski/linux/./tools/net/ynl/cli.py", line 100, in main
      reply = ynl.dump(args.dump, attrs)
    File "/home/kicinski/linux/tools/net/ynl/lib/ynl.py", line 1064, in dump
      return self._op(method, vals, dump=True)
    File "/home/kicinski/linux/tools/net/ynl/lib/ynl.py", line 1058, in _op
      return self._ops(ops)[0]
    File "/home/kicinski/linux/tools/net/ynl/lib/ynl.py", line 1045, in _ops
      rsp_msg = self._decode(decoded.raw_attrs, op.attr_set.name)
    File "/home/kicinski/linux/tools/net/ynl/lib/ynl.py", line 738, in _decode
      subdict = self._decode(NlAttrs(attr.raw), attr_spec['nested-attributes'], search_attrs)
    File "/home/kicinski/linux/tools/net/ynl/lib/ynl.py", line 763, in _decode
      decoded = self._decode_sub_msg(attr, attr_spec, search_attrs)
    File "/home/kicinski/linux/tools/net/ynl/lib/ynl.py", line 714, in _decode_sub_msg
      subdict = self._decode(NlAttrs(attr.raw, offset), msg_format.attr_set)
    File "/home/kicinski/linux/tools/net/ynl/lib/ynl.py", line 749, in _decode
      decoded = attr.as_scalar(attr_spec['type'], attr_spec.byte_order)
    File "/home/kicinski/linux/tools/net/ynl/lib/ynl.py", line 147, in as_scalar
      return format.unpack(self.raw)[0]
  struct.error: unpack requires a buffer of 2 bytes

The Traceback is what we would previously see, the "Error..."
messages are new. We print a message per level (in the stack
order). Printing single combined message gets tricky quickly
given sub-messages etc.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250107022820.2087101-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>