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
- add test to confirm error is raised and message is correct

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] || 'normal'}) is not a known font."
         end
 
         super

spec/prawn/font_spec.rb

@@ -69,6 +69,14 @@
       end
     end
 
+    it 'reports missing font with style' do
+      expect {
+        pdf.font('Nada', style: :bold) do
+          pdf.width_of('hello')
+        end
+      }.to raise_error(Prawn::Errors::UnknownFont, /Nada \(bold\)/)
+    end
+
     it 'calculates styled widths correctly using TTFs' do
       pdf.font_families.update(
         'DejaVu Sans' => {