Help users to number activities consecutively

[hofstef]

Is your feature request related to a problem? Please describe.
Sometimes one needs to rearrange, delete or insert activities in a domain story. This will probably mess up the numbering.

Describe the solution you'd like
To check if the activities are in the right sequence, I replay the domain story. But that requires the activities to be numbered consecutively. If they are not, a error message is shown. I would like a error message that tells me which numbers are missing.

Describe alternatives you've considered
The error message could also show which is the last number that is still ok.