When splitting a signal, show user-friendly error messages

[bartjkdp]

Describe the bug
When splitting a Signal with a source that is not available any more, the UI only shows "An unexpected error has occured"

To Reproduce
Steps to reproduce the behavior:

1. Create a Signal under a specific source
2. Remove the source
3. Split the signal
4. See error

Expected behavior
It would be great if the frontend shows a user-friendly error message like: "The source of the original Signal is not available any more"