Optional line numbers for liquid errors

lib/liquid/errors.rb

@@ -1,5 +1,35 @@
 module Liquid
-  class Error < ::StandardError; end
+  class Error < ::StandardError
+    attr_accessor :line_number
+
+    def self.render(e)
+      msg = if e.is_a?(Liquid::Error) && e.line_number
+        "#{e.line_number}: #{e.message}"
+      else
+        e.message
+      end
+
+      case e
+      when SyntaxError
+        "Liquid syntax error: #{msg}"
+      else
+        "Liquid error: #{msg}"
+      end
+    end
+
+    def self.error_with_line_number(e, token)
+      if e.is_a?(Liquid::Error)
+        e.set_line_number_from_token(token)
+      end
+
+      e
+    end
+
+    def set_line_number_from_token(token)
+      return unless token.respond_to?(:line_number)
+      self.line_number = token.line_number
+    end
+  end
 
   class ArgumentError < Error; end
   class ContextError < Error; end