Ensure only valid claims can be completed

In investigating #87 I noticed that there was no validation that a claim existed and was valid when it was completed.

I have asked some questions in #90 to clarify what the intended behavior is, but I believe that it shouldn't be possible to complete a claim that is not currently valid.
Correct; there's no way to do this validation without coordination, which would reduce availability. Moreover, it's not like the validation would make anything more safe; all it would do is destroy information.