cubicweb-seo #4021594 generate sitemap even if some rules have failed [validation pending]

if an exception in throw during a rule, the sitemap is not generated.

priorityimportant
typebug
done in0.2.1
load0.200
load left0.000
closed by#04a6014760d4 make sitemap view resilient to errors in sitemap rules (closes #4021594)
patchmake sitemap view resilient to errors in sitemap rules (closes #4021594) [applied]