Display value when raising due to unscope() issues

Hopefully make it easier to debug errors. e.g

Before:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String is unimplemented.

After:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String "'t'='t'" is unimplemented.

activerecord/lib/active_record/relation/query_methods.rb

@@ -877,7 +877,7 @@ def where_unscoping(target_value)
           subrelation = (rel.left.kind_of?(Arel::Attributes::Attribute) ? rel.left : rel.right)
           subrelation.name == target_value
         else
-          raise "unscope(where: #{target_value.inspect}) failed: unscoping #{rel.class} is unimplemented."
+          raise "unscope(where: #{target_value.inspect}) failed: unscoping #{rel.class} \"#{rel}\" is unimplemented."
         end
       end