Rephrase exception message when unable to combine branches

[hampusboesenfunnel]

If the function main._combine_branches fails to combine branches, it raises a ValueError with this message:

Multiple branches dont know how to handle these, either

I think this message can be improved to better communicate the context in which the problem arises.

Suggestions

• Include the selected method in the message
• Include the branches that couldn't be processed in the message (this can potentially be quite long, but if at least the point(s) of branching can be communicated I think it would be quite helpful)
• I don't quite understand what the "either" means in the context of this exception—maybe it can be rephrased or omitted?

What do you think about something like this?

Unable to combine branches with the selected method:
    method: None
    branches: {'a': {'A': 123}, 'b':[{'c':1},{'c':2}]}