handle celery worker limit

[gubuntu]

problem

currently max seven concurrent analysis can run (each in their own celery worker). If you want to start another analysis in that situation then you either have to wait for one to finish or cancel one.

However, a user would have no way of knowing this.

solution

communicate to the user if this situation arises as well as what they can do about it

[gubuntu]

from @lucernae in Slack:

But it would make sense later if GeoSAFE automatically limit maximum concurrent jobs based on total scale that we used. It’s just that, we don’t have a way for GeoSAFE to automatically know that limit yet. It had to be set manually if we want to implement it.