Show input in error message

Show date input in error message to make it easier for user
to find infringing date/file

lib/jekyll/utils.rb

@@ -89,10 +89,10 @@ module Jekyll
       #
       # Returns the parsed date if successful, throws a FatalException
       # if not
-      def parse_date(input, msg = "Invalid date: could not be parsed.")
+      def parse_date(input, msg = "Input could not be parsed.")
         Time.parse(input)
       rescue ArgumentError
-        raise Errors::FatalException.new(msg)
+        raise Errors::FatalException.new("Invalid date '#{input}': " + msg)
       end
 
     end