feature #2371: raise exception and cli error when nokogiri is not installed

src/cli/one_helper.rb

@@ -16,7 +16,13 @@
 
 require 'cli_helper'
 
-require 'opennebula'
+begin
+    require 'opennebula'
+rescue Exception => e
+    puts "Error: "+e.message.to_s
+    exit(-1)
+end
+
 include OpenNebula
 
 module OpenNebulaHelper

src/oca/ruby/opennebula/xml_utils.rb

@@ -129,5 +129,7 @@ module OpenNebula
                 alias :cdata_block :characters
             end
         end
+    else
+        raise "nokogiri gem not installed."
     end
 end