point out the backup if the upgrade fails

From Bruce Korb on the mailing list.

I was trying to follow directions:

$ bzr update
Working tree format 3 is deprecated and a better format is available.
It is recommended that you upgrade by running the command
  bzr upgrade file:///share/st/devbk/simple-cdd-0.3.5/
Tree is up to date at revision 405.
$ bzr upgrade file:///share/st/devbk/simple-cdd-0.3.5/
starting upgrade of file:///share/st/devbk/simple-cdd-0.3.5/
making backup of tree history
file:///share/st/devbk/simple-cdd-0.3.5/.bzr has been backed up to
file:///share/st/devbk/simple-cdd-0.3.5/.bzr.backup
if conversion fails, you can move this directory back to .bzr
if it succeeds, you can remove this directory if you wish
bzr: ERROR: Could not acquire lock [Errno 37] No locks available
/usr/lib/python2.4/site-packages/bzrlib/lock.py:79: UserWarning: lock
on <open file u'/share/st/devbk/simple-cdd-0.3.5/.bzr/checkout/dirstate',
mode 'wb+' at 0xb7c3a0f8> not released
  warn("lock on %r not released" % self.f)
=====================
At this point, I did a number of things to investigate why I could not
get the lock. It turns out, I did a ``yum install'' as root (because you
have to do that as root) and so the lock files were owned by and
only modifiable by root. Oops. (Not your issue, tho.) Meanwhile,
the ``if conversion fails, you can move this directory back'' comment
was pages and pages back in my scroll history and not quite scrolled
off yet. Good thing because future attempts:
======================
$ bzr upgrade file:///share/st/devbk/simple-cdd-0.3.5/
starting upgrade of file:///share/st/devbk/simple-cdd-0.3.5/
making backup of tree history
bzr: ERROR: File exists:
u'/share/st/devbk/simple-cdd-0.3.5/.bzr.backup': [Errno 17] File
exists: '/share/st/devbk/simple-cdd-0.3.5/.bzr.backup'
======================
don't give you that hint. Hint: it would be a good idea to repeat the hint :)
It would have avoided this missive.

Thanks - Bruce