409 error should display a better error message

[mcglincy]

Right now I see a "Could not create project: Unexpected response status 409". This is likely due to a workspace/slug name collision, so we could show a message to that effect. E.g., "Could not create project: / is already in use."

[CobusT]

I just encountered this. There was a previous project with the same slug that was deleted. When I try and create a new project with that slug I get this error.

If we show a better error message, then we should keep in mind that the user may not see a project with that name in the workspace anymore (since the slug is occupied by a deleted project).

Alternatively we should allow the slug to be re-used.

[mbostock]

Yes, we should allow the slug to be re-used.