Reinstate a negative test to check that the format exception is caught in AnalyzeSkeletonWrapper

[alessandrofelder]

We used to have a negative test to ensure the format exception is caught as expected by the AnalyzeSkeletonWrapper, and that the correct error message relayed to the user in this case. See e.g. here for an idea how it worked. However, the way we mocked a File with a faulty format has become out of date. We should reinstate this test as soon as we figure out how to mock a File with a faulty format well.

[mdoube]

#283 (comment)