tmrca error when common ancestor does not exist

[daikitag]

When doing ancestry simulations through the fixed_pedigree model, it is possible to have no common ancestor, as shown in the diagram below. However, in this case, we get an uninformative error when running tmrca function. Since some people might be interesting in writing a for loop to examine the time to the MRCA for each node combination, would it be possible to return np.inf or an informative error when this happens?

[jeromekelleher]

I thought we'd fixed something like this recently - are you on the most recent tskit version @daikitag?

[daikitag]

@jeromekelleher Sorry, I was using version 0.5.5, and it seems that an informative error is coming out.