Support starting a transaction with a non-existent repository

Beginning a transaction currently fails if the repository does not exist. This is generally fine for RPCs that would attempt to operate on the repository by doing reference updates for example. The handler would anyway error out if the repsitory doesn't exist. Some RPCs however should run even if the repository doesn't exist. For example, RepositoryExists returns whether the repository exists or not in its response. The various repository creating RPCs also should run even if the repository doesn't exist. To accommodate for those RPCs, this commit allows starting a transaction even if the repository doesn't exist. The RPC handlers anyway check for repository existence, so it's fined to call them without a repository.

Tests are updated to verify the repository doesn't exist throgh a separate assertion where previously they relied on a not found error being returned.