bug#20785: user-error for mark-inactive

[Juri Linkov]

>> A single remaining case that is too annoying is when the debugger is
>> called due to a user error while accidentally typing `C-w' (kill-region)
>> when there is no active mark in the buffer:
>
> Looks OK to me

Installed.

> (maybe we could also just signal `mark-inactive' instead
> of calling user-error, but either way is OK for me),

I see that `mark-inactive' and user-error in `kill-region'
have different messages:

`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"

and I don't see how to merge these messages.