If a conflict prevents saving, error shows "[object object]"

[sleidig]

Describe the bug
If a conflict occurs, error shown to user is not readable or understandable

To Reproduce
Steps to reproduce the behavior:

1. Open a synced instance (e.g. dev.aam-digital.com) in two different browsers
2. Open the same child's details page in both windows and click Edit in both
3. change the name in both windows
4. click "save" in the first window - it will save successfully
5. click "save" in the second window - it will fail, showing an alert with a cryptic message

Expected behavior
Give a readable error message in language a normal user can understand.
Give guidance on what the user can do to fix the problem (reload the page and try again?).