improve the error message fetch returns if a checkout exists

fetch.py

Index: fetch.py
diff --git a/fetch.py b/fetch.py
index 1e5d688f0b177f1d3dcf19c0ba8d4260472d84fa..d2b37da4a2a75b64b76f16d653a546c1a5c121f0 100755
--- a/fetch.py
+++ b/fetch.py
@@ -247,8 +247,11 @@ def run(dryrun, 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()