tools: ynl: replace print with NlError

Instead of dumping the error on the stdout, make the callee and
opportunity to decide what to do with it. This is mostly for the
ethtool testing.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/ynl.py

@@ -67,6 +67,14 @@ class Netlink:
     NLMSGERR_ATTR_MISS_NEST = 6
 
 
+class NlError(Exception):
+  def __init__(self, nl_msg):
+    self.nl_msg = nl_msg
+
+  def __str__(self):
+    return f"Netlink error: {os.strerror(-self.nl_msg.error)}\n{self.nl_msg}"
+
+
 class NlAttr:
     type_formats = { 'u8' : ('B', 1), 's8' : ('b', 1),
                      'u16': ('H', 2), 's16': ('h', 2),
@@ -551,9 +559,7 @@ class YnlFamily(SpecFamily):
                     self._decode_extack(msg, op.attr_set, nl_msg.extack)
 
                 if nl_msg.error:
-                    print("Netlink error:", os.strerror(-nl_msg.error))
-                    print(nl_msg)
-                    return
+                    raise NlError(nl_msg)
                 if nl_msg.done:
                     if nl_msg.extack:
                         print("Netlink warning:")