[BUG] improve error messages

[Jari Aalto]

any operation that is tried on directory which does not contain ./.bzr
directory now results (I tried mistakenly 'add'):

bzr: ERROR: /cygdrive/e/home/jaalto/tmp/vc/bzr/2 is not in a branch
  at /usr/src/build/build/bazaar-ng/bzr-0.0.7/.inst/usr/share/lib/python2.4/site-packages/bzrlib/branch.py line 131, in find_branch_root()

Could the error handling be improved a little so that it would first 
check the existence of ./.bzr and say:

  $ bzr add
  bzr: ERROR: Directory is not bzr controlled (no .bzr/ found; run bzr init)

Jari