Workflow interoperability

[Trello](https://trello.com/c/xAsq2mNn/11-buildstep-interop)
Yes, of course. On the other hand such errors would be obvious if they occurred.

I did try to think about ways to make this logic work with other `Run`s but it did not seem practical, as you need to know which workspace a line of text applied to, and then have a permalink to it. This hyperlinking anyway only works in the limited case that the workspace is preserved from the time the build failed until the time the mail is read and the link clicked; it is only “best effort”.