Code review comment for lp:~jimbaker/pyjuju/hook-command-error-cases

I'm fine with reporting "Relation not found", however, according to the spec, it should report "Not a valid relation id".

Note that the format check was being done on the other get_relation_hook_context (unfortunate naming) in HookContext; however, the invoker always caches associated hook contexts, then looks them up, so this check was never actually being made at a user visible level.