- Now if a rewriteFile fails, we'll get a stack trace on -v.
  We should rewrite writeFileAtomic but it's in the wrong
  module and it would involve changing a lot of clients, so
  this is a more conservative change.

- We no longer wrap error messages in top handler, so that
  the stacks look better.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>