compactor adjustments v2

[Tomi Ollila]

This is v2 of id:1384192538-15291-1-git-send-email-tomi.ollila at iki.fi 

Changes include: 
  tidy formatting (i.e. "uncrustifying") on related code
  catching Xapian::Error (always)

The tests done in  id:m2fvr1tpkf.fsf at guru.guru-group.fi  were redone
with one extra (*) -- results same. Automatic tests pass.

(*)

$ mkdir xapian.compact
$ touch xapian.compact/file
$ chmod 000 xapian.compact
$ notmuch compact 
Compacting database...
compacting table postlist
Error while compacting: Couldn't open base /path/to/.notmuch/xapian.compact/postlist.baseA to write: Permission denied
Compaction failed: A Xapian exception occurred
zsh: exit 1     ./notmuch compact