Log: executor: improve resource registration/unregistration messages

Example:

info: Resource 'Fencing' not found (0 active resources)
info: Added 'Fencing' to the rsc list (1 active resources)

->

info: Agent information for 'Fencing' not in cache
info: Cached agent information for 'Fencing'

I think the original messages could be confusing as to whether there was
something wrong with the resource itself, and what "active" and "rsc list"
meant.