add a warning message and a new file to write out missing files

[kaitejohnson]

This PR adds the functionality to generate warning messages when our "roll-your-own" version of tar_combine() (called combine_outputs() tries to load in a file and fails. This is helpful because some of these seem to have been killed jobs on Azure that we would only know by spot checking manually (which is not ideal) and we would just need to rerun them.

I added functionality to write it into a file, we could make this optional. My thinking here is most of this analysis is really for our own evaluation analysis rather than very general tooling, so its maybe okay to do something like this, but happy to change if we want to push towards maximizing user generality.

[dylanhmorris]

Would be nice to have a unit test for this function to confirm it does indeed create the file as needed, but that can be a separate PR. Make an issue?