Repositories backed by hosting services could crash both the repository

list in the administration UI and the repository configuration form.

Assumptions were being made about whether a HostingService could be

loaded, and this meant we weren't catching exceptions.
The repository list now catches this case when trying to render the

hosting service name, and falls back on showing the stored ID instead,

along with a "(missing support)" suffix.
The repository configuration form catches this in a couple of spots as

well, allowing the form to load even if a hosting service can't.
There are still problems that can occur with rendering information for

SCMTools, due to the fact that they can be loaded from entrypoints, but

those are beyond the scope of this change.

Tested loading my repository list and config form while Power Pack was

uninstalled. Saw that both pages successfully loaded, and that the

affected repositories had the new suffix.