improve the error message fetch returns if a checkout exists

fetch.py

 #!/usr/bin/env python
 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 Tool to perform checkouts in one easy command line!
 
 Usage:
   fetch <recipe> [--property=value [--property2=value2 ...]]
@@ skipping 229 matching lines @@
       root: The directory into which the repo expects to be checkout out.
   """
   assert 'type' in spec
   checkout_type = spec['type']
   checkout_spec = spec['%s_spec' % checkout_type]
   try:
     checkout = CheckoutFactory(checkout_type, dryrun, checkout_spec, root)
   except KeyError:
     return 1
   if checkout.exists():
-    print 'You appear to already have this checkout.'
-    print 'Aborting to avoid clobbering your work.'
+    print 'You appear to already have a checkout. "fetch" is used only'
+    print 'to get new checkouts. Use "gclient sync" to update the checkout.'
+    print
+    print 'Fetch also does not yet deal with partial checkouts, so if fetch'
+    print 'failed, delete the checkout and start over (crbug.com/230691).'
     return 1
   return checkout.init()
 
 
 def main():
   dryrun, recipe, props = handle_args(sys.argv)
   spec, root = run_recipe_fetch(recipe, props)
   return run(dryrun, spec, root)
 
 
 if __name__ == '__main__':
   sys.exit(main())