[Ur] Exceptions handling

[Adam Chlipala]

Vladimir Shabanov wrote:
> So savepoint will be introduced only where it really needed (dml with
> user error handling)? I think that this is the best solution.
>    

Yes, that's what I meant.

> BTW it can be useful to have function for cancelling (and maybe
> restarting) transaction as a part of error handling.
>    

In Ur/Web, transactions are used for all state, not just the database.  
I would want to have a reasonable integration of transaction restart 
with all kinds of state, which can include arbitrary protocols thanks to 
C FFI libraries.  That would complicate the FFI, so I'll leave this out 
for now.  If a compelling use case comes up, let me know.