Policy validation generates an unhelpful error when "size" doesn't contain a unit

[dseevr]

What I ran:
docker volume create -d volcontiv --name $POLICY/$NAME --opt size=10

What I expected:
A helpful error message telling me that my input was not valid. It should say that the "size" must contain units (e.g., "10MB").

What I got:
An unhelpful error message: Size set to zero for non-empty CRUD backend ceph

cc: @neelimamukiri