[Ur] Exceptions handling

[Adam Chlipala]

OK, I've pushed some changes to the public Mercurial repository linked 
from the Ur front page.

There are two main changes relevant to this thread, based on the two 
main suggestions discussed here:
- A function Basis.tryDml, for running DML in a way such that error 
messages can be processed programmatically.
- A .urp directive onError, for setting a function of type [xbody -> 
transaction page] for generating 500 error pages from error messages.

Both of these are further documented in the version of the manual found 
in the repository.