Exception handling for type: relation

It would appear many plugins incorrectly use type: relation as a list column, when this does nothing. Previously it would fallback to the text type, now that invalid types fail hard, this adds a softer landing by spamming the trace log instead.
Refs #2438

modules/backend/widgets/Lists.php

@@ -920,6 +920,16 @@ class Lists extends WidgetBase
         return $this->evalTextTypeValue($record, $column, $value);
     }
 
+    /**
+     * Common mistake, relation is not a valid list column.
+     * @deprecated Remove if year >= 2018
+     */
+    protected function evalRelationTypeValue($record, $column, $value)
+    {
+        traceLog(sprintf('Warning: List column type "relation" for class "%s" is not valid.', get_class($record)));
+        return $this->evalTextTypeValue($record, $column, $value);
+    }
+
     /**
      * Process as partial reference
      */