bug#20785: user-error for mark-inactive

[Stefan Monnier]

> `mark-inactive' signaled in the function `mark'
> has the message only about the mark:

>   "The mark is not active now"

> whereas the user-error message in `kill-region'
> is specific about the region:

>   "The mark is not set now, so there is no region"

I'm not sure how important is this difference (or even if it's intentional).


        Stefan