knowledge_pad: add missing error_message workflow variable

bt5/erp5_knowledge_pad/WorkflowTemplateItem/portal_workflow/knowledge_pad_validation_workflow/variable_error_message.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="Workflow Variable" module="erp5.portal_type"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>automatic_update</string> </key>
+            <value> <int>1</int> </value>
+        </item>
+        <item>
+            <key> <string>description</string> </key>
+            <value> <string>Error message if validation failed</string> </value>
+        </item>
+        <item>
+            <key> <string>for_catalog</string> </key>
+            <value> <int>0</int> </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>variable_error_message</string> </value>
+        </item>
+        <item>
+            <key> <string>portal_type</string> </key>
+            <value> <string>Workflow Variable</string> </value>
+        </item>
+        <item>
+            <key> <string>status_included</string> </key>
+            <value> <int>1</int> </value>
+        </item>
+        <item>
+            <key> <string>title</string> </key>
+            <value>
+              <none/>
+            </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>