erp5_web_js_style: do not crash if style gadget can't be loaded

Try to support displaying web js site from google cache,
which can not access original gadget, due to cross origin ajax query forbidden.

Display original content instead and load the default CSS in such case.
11 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Master

00:51:43

passed ERP5.PerformanceTest-Master

00:22:55

failed ERP5.UnitTest-Master

02:16:09

failed ERP5.UnitTest-Master.Medusa

01:13:30

passed SlapOS.Eggs.UnitTest-Master.Python2

00:08:38

passed SlapOS.Eggs.UnitTest-Master.Python3

00:08:06

failed ERP5.UnitTest-Master

01:33:01

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:25

passed SlapOS.Eggs.UnitTest-Master.Python2

00:19:42

passed SlapOS.Eggs.UnitTest-Master.Python3

00:09:05

passed SlapOS.Eggs.UnitTest-Master.Python3

00:20:28