cubicweb #3670208 when some source using unknown source type is encountered, can't even start a shell [validation pending]

which is not convenient to fix the pb. The pb should be reported and the source skipped.

prioritynormal
typebug
done in3.19.0
load0.100
load left0.000
closed by#9fb2f15d5e85 [sources] Skip problematic sources when starting shell instead of crashing. Closes #3670208
patch[sources] Skip problematic sources when starting shell instead of crashing. Closes #3670208 [applied]