Fix error message when nominating a backport via the script

* tools/dist/nominate-backport.py
  (): We use backport.wc so better import it
  (main): Expect stderr W170000 (URL non-existent) when calling svn info to
          check for a backport branch - this means no branch exists.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1936153 13f79535-47bb-0310-9956-ffa450edef68

Author: d-sahlberg

tools/dist/nominate-backport.py

@@ -39,6 +39,7 @@
 
 import backport.merger
 import backport.status
+import backport.wc
 
 # Constants
 STATUS = './STATUS'
@@ -166,7 +167,8 @@ def main():
 
   # Determine whether a backport branch exists
   (exit_code, branch, stderr) = backport.merger.run_svn(['info', '--show-item', 'url', '--',
-                              wcinfo["URL"] + '-r'+str(revisions[0])]).replace('\n', '')
+                              wcinfo["URL"] + '-r'+str(revisions[0])], 'W170000')
+  branch.replace('\n', '')
   if branch == "":
     branch = None