fix ticket #1382716 (problem was actually more subtle than I originally thought)
* log errors happening in looping tasks and repo threads
* do not swallow RegistryNotFound exceptions which occur during hook execution (only those happening during hook registry selection)
[html generation] even when doctype without extension are asked, we should define the xmlns:cubicweb attribute to avoid html validation error during tests (break tests with new calendar view in default branch since it demote to html)