Include element parsing errors in final diff results instead of failing

**Is your feature request related to a problem? Please describe.**

Recently working with @natanrolnik we noticed running `xcdiff` fails due to an incorrect path in one of the projects, thanks to #64 the source of the error was identified, however sadly as it wasn't immediately actionable it resulted in the inability to see the diff results for the remaining elements which were needed.

**Describe the solution you'd like**

Potentially log errors or collect errors and post them in the final diff result while continuing to perform a diff on other elements. This allows users to see a best effort diff while clearly seeing some of the elements which could not be diffed.

**Describe alternatives you've considered**

- Ignore errors, however this could yield false results (e.g. no diff if all elements fail to parse for any reason) as such is probably not a suitable alternative.