nuke validate backdrop with help

openpype/hosts/nuke/plugins/publish/help/validate_backdrop.xml

@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<root>
+    <error id="multiple_outputs">
+        <title>Found multiple outputs</title>
+        <description>
+## Invalid output amount
+
+Backdrop is having more than one outgoing connections.
+
+### How to repair?
+
+1. Use button `Center node in node graph` and navigate to the backdrop.
+2. Reorganize nodes the way only one outgoing connection is present.
+3. Hit reload button on the publisher.
+        </description>
+        <detail>
+### How could this happen?
+
+More than one node, which are found above the backdrop, are linked downstream or more output connections from a node also linked downstream.
+        </detail>
+    </error>
+    <error id="no_nodes">
+        <title>Empty backdrop</title>
+        <description>
+## Invalid empty backdrop
+
+Backdrop is empty and no nodes are found above it.
+
+### How to repair?
+
+1. Use button `Center node in node graph` and navigate to the backdrop.
+2. Add any node above it or delete it.
+3. Hit reload button on the publisher.
+        </description>
+    </error>
+</root>

openpype/hosts/nuke/plugins/publish/validate_backdrop.py

@@ -1,6 +1,7 @@
 import nuke
 import pyblish
 from openpype.hosts.nuke.api.lib import maintained_selection
+from openpype.pipeline import PublishXmlValidationError
 
 
 class SelectCenterInNodeGraph(pyblish.api.Action):
@@ -63,8 +64,20 @@ class ValidateBackdrop(pyblish.api.InstancePlugin):
         msg_multiple_outputs = (
             "Only one outcoming connection from "
             "\"{}\" is allowed").format(instance.data["name"])
-        assert len(connections_out.keys()) <= 1, msg_multiple_outputs
 
-        msg_no_content = "No content on backdrop node: \"{}\"".format(
+        if len(connections_out.keys()) > 1:
+            raise PublishXmlValidationError(
+                self,
+                msg_multiple_outputs,
+                "multiple_outputs"
+            )
+
+        msg_no_nodes = "No content on backdrop node: \"{}\"".format(
             instance.data["name"])
-        assert len(instance) > 1, msg_no_content
+
+        if len(instance) == 0:
+            raise PublishXmlValidationError(
+                self,
+                msg_no_nodes,
+                "no_nodes"
+            )