InvalidPropertyPrefix error class

Raise a InvalidPropertyPrefix error if the prefix is invalid.

lib/microformats2.rb

@@ -33,4 +33,7 @@ module Microformats2
       html
     end
   end # class << self
+
+  class InvalidPropertyPrefix < StandardError; end
+
 end

lib/microformats2/format.rb

@@ -37,6 +37,7 @@ module Microformats2
       prefix = property_class.split("-").first
       # find ruby class for kind of property
       klass = Microformats2::Property::PREFIX_CLASS_MAP[prefix]
+      raise InvalidPropertyPrefix unless klass
       # We don't have a nokogiri element so pass in nil
       property = klass.new(nil, property_class, value)
       assign_property(property)