Support displaying the unavailable worker version error when starting a process

The error message implemented in #1406 when starting a process with an unavailable worker version is displayed like so:

But we already have some better handling for error messages here, at least on the missing model error:

The unavailable worker version error should be properly displayed, to let the user know what is actually going on. Implementing #1204 (closed) will make this error less likely to appear, but it could still be possible to get it, so it should be displayed.