The --teamcity option silently fails if the extension is not installed

[CharliePoole]

Based on the code as well as my memory, this was intentional. Assuming the user is running interactively, it makes sense to just go ahead and run and hopefully it will be clear that the extension is missing. But of course --teamcity is not normally used interactively but when running on a CI server.

It now seems better to me to give an error message and stop the run if the extension is not available.

@OsirisTerje @veleek @NikolayPianikov Does that make sense to you?