I ended up writing custom interface for my app, which included custom controller actions that returned readable errors for validation.
Kudos to Luke Murphey, whose Splunk Web Input really helped me understand how to write custom UI for the app.
I think external validation is useless if you can't tell the user what is the reason of failure. This seems like a huge design overlook for modular inputs. On the other hand, it's great that Splunk allows to tune the user interface to your needs - even though there is hardly any documentation on how to do it.
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