resolves prawnpdf#1022 provide more information about missing font

- when an afm font is missing, provide more information to the user
- look for font name in family option in case of missing TTF

lib/prawn/font/afm.rb

@@ -49,8 +49,9 @@ def self.font_data
       end
 
       def initialize(document, name, options = {}) #:nodoc:
+        name ||= options[:family]
         unless BUILT_INS.include?(name)
-          raise Prawn::Errors::UnknownFont, "#{name} is not a known font."
+          raise Prawn::Errors::UnknownFont, "#{name} (#{options[:style] || 'regular'}) is not a known font."
         end
 
         super