[Ur] Exceptions handling

[Vladimir Shabanov]

Hello.

Ur/Web can throw error on invalid DML (for example "ERROR: duplicate
key value violates unique constraint ..."). Currently this error
results in invalid page.

Is there a way to handle such errors?

I've found some error handling related functions (onError, onFail,
etc.) in basis.urs but they are client-side-only. And I haven't found
any example of how they can be used.