Make it clear the exploit doesn't like certain PDF formats

If the exploit cannot fetch certain xref fields, we warn the user
we don't like their PDF, and recommend them to try a different
one.

modules/exploits/windows/fileformat/adobe_pdf_embedded_exe.rb

@@ -79,10 +79,14 @@ class Metasploit3 < Msf::Exploit::Remote
 		startxrefs	= pdf_objects[2]
 		root_obj	= pdf_objects[3]
 
+		begin
 			output = basic_social_engineering_exploit(xref_trailers,root_obj,stream,trailers,file_name,exe_name,startxrefs.last)
 
 			print_status("Creating '#{datastore['FILENAME']}' file...")
 			file_create(output)
+		rescue KeyError => e
+			print_error("Incompatible PDF structure: #{e.message}. Please try a different PDF.")
+		end
 	end