[MERGE] Log smart server errors in the trace file

[Martin Pool]

Martin Pool has voted +1 (conditional).
Status is now: Conditionally approved
Comment:
It probably ought to be mentioned in NEWS that the smart server now logs 
exceptions to the trace file.

As a followon, bzr serve should probably have an option to send the log 
elsewhere.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20070717070522.GG27285%40steerpike.home.puzzling.org%3E