Add line column to message

Useful for `map(&:to_s)` and simple output

lib/nokogiri/xml/syntax_error.rb

@@ -40,7 +40,7 @@ def fatal?
       end
 
       def to_s
-        super.chomp
+        [super.chomp, line, column].compact.join(', ')
       end
     end
   end

test/xml/test_entity_reference.rb

@@ -141,7 +141,7 @@ def setup
         end
         assert_kind_of Nokogiri::XML::EntityReference, doc.xpath('//body').first.children.first
         if Nokogiri.uses_libxml?
-          assert_equal ["Entity 'bar' not defined"], doc.errors.map(&:to_s)
+          assert_equal ["Entity 'bar' not defined, 5, 14"], doc.errors.map(&:to_s)
         end
       end
 
@@ -159,7 +159,7 @@ def setup
         end
         assert_kind_of Nokogiri::XML::EntityReference, doc.xpath('//body').first.children.first
         if Nokogiri.uses_libxml?
-          assert_equal ["Attempt to load network entity http://foo.bar.com/", "Entity 'bar' not defined"], doc.errors.map(&:to_s)
+          assert_equal ["Attempt to load network entity http://foo.bar.com/, 0, 0", "Entity 'bar' not defined, 4, 34"], doc.errors.map(&:to_s)
         else
           assert_equal ["Attempt to load network entity http://foo.bar.com/"], doc.errors.map(&:to_s)
         end

test/xml/test_syntax_error.rb

@@ -25,6 +25,14 @@ def reader.errors
           reader.read
         }
       end unless Nokogiri.jruby? # which does not internally call `errors`
+
+      def test_line_column
+        bad_doc = Nokogiri::XML('test')
+        error = bad_doc.errors.first
+        assert_equal "Start tag expected, '<' not found, 1, 1", error.message
+        assert_equal 1, error.line
+        assert_equal 1, error.column
+      end
     end
   end
 end